I mean, we've had exclusively Democrat or Republican governments for well over 150 years, I'm not sure how else you'd measure "total and perpetual dominance".

Let's imagine that this were to actually happen. Major members of the two parties realized that what you said was true, and that if they could cooperate then they would totally and completely dominate the political system, and could do whatever they wanted, no matter what the electorate said, and no matter how bad it was for those governed.

They would soon realize that there was a problem, in that the electorate like to have a choice, and would unquestionably form some sort of opposition to any government that tried to rule without offering a choice.

The solution they would come up with, in order to make this dominance happen, would be to offer the illusion of choice. So they would agree to offer the electorate two alternatives, ensuring their continued dominance in two ways:

1. Making the two 'alternatives' actually very similar. So rather than offering options that might be popular in other countries, such as increased healthcare or higher wages, offer two choices that differ only very slightly from each other.


2. Keep up the appearance of strong enmity between the parties, and couple it with a very complex and oppositional legislative system, with no way of resolving disputes if two parts of the legislature disagree. This ensures that there is always a good reason not to pass laws or make changes, no matter how popular. It's always the fault of that pesky other party that nothing can be done.

So that's what would probably happen if this was tried. I leave any conclusions to the reader.

I actually upvoted K Dog's answer because "uniparty" is probably the hypothesis being advanced by the question (which itself is rather vague). But it's also easy to see why doesn't really work that way: primaries. It's easy for a party to change is position over time when its candidates are elected too, and that happens in the primaries. Just look how the Republican part of the "uniparty" got turned toward a new discourse by the Trump takeover. Not long before that was the Tea Party etc.

The "uniparty" only lasts as long as there's not a strong enough grassroots base for some divergent idea to be embraced by candidates in the primaries of one of the parties (but not the other). And there can be many such issues causing a divergence. And empirical data nicely shown in another question here actually points to a divergence rather than converges of the two main parties on ideology. The graph in the question linked is only about Congress, but there's separate data that that divergence really comes from the public divergence (i.e. it's not just some fight between two small cliques of politicians):

So you can't get a "uniparty" from that trend... unless the elections get subverted somehow.

The importance of the primaries in the US for the good functioning of the democracy as really representative is emphasized by the laws that provide public funding for primaries, etc., in order to prevent them from being a closed club.

In the US, the two parties comparative to most of Europe are not ideologically far apart. First-past-the-post elections tend to concentrate on winning the "independent" voter which has a tempering impact on ideological shift. What you are referring to has a name; it's called the uniparty that is generally benefits from Pax Americana and is globalist, free-trade, open-borders, and definitely pro-spending. Here's a snippet on the uniparty.

The Republican Party’s leaders have functioned as junior members of America’s single ruling party, the UniParty. Acting as the proverbial cockboat in the wake of the Democrats’ man-of-war, they have made Democratic priorities their own when the White House and the Congress were in the hands of Republicans as well as in those of Democrats, and when control has been mixed. The UniParty, the party of government, the party of Ins, continues to consist of the same people. The Outs are always the same people too: American conservatives. They don’t have a party.

Whatever differences exist within the Uniparty, between Republican John Boehner and Democrat Nancy Pelosi, between Republican Mitch McConnell and Democrat Harry Reid, get worked out behind closed doors. Those differences are narrow. The latest negotiations were over some $80 billion out of three trillion dollars in spending. The bipartisan negotiators did not let into the room any of the major issues that concern Americans. Not Obamacare, not racial preferences, not religious liberty, not endless no-win wars. The UniParty is unanimous: more of the same!

Here's some more from Politico

“The Uniparty” is the latest populist buzzword to seize the imagination of the drain-the-swamp crowd, those who see grand conspiracies in the machinations of the “deep state” and globalist-corporate forces. It has a crisp clarity, instantly conveying the idea of an establishment cabal, Democrat and Republican alike, arrayed against their outsider hero, Donald Trump.

But while “the Uniparty” may be trendy among the Breitbart set, it wasn’t born there. In fact, if you go back to the contentious presidential race of 2000, you’ll find it arose as a political barb among supporters of Ralph Nader, running as the nominee of the Green Party.

Numerous posts on the Usenet newsgroup alt.politics.green from that year railed against “the two-headed UniParty,” “the money-driven media/political uniparty environment,” “the corporate Uniparty grip on the civic polity,” and so forth.

This article also is really good

The mathematical steady-state for any voting system is a number of parties roughly inversely proportional to the amount of the vote it takes to get elected. In a national plurality system, that works out to 1 / 0.50, or 2. This is known as Duveger's Law.

Historically you can see this. There have been multiple times in US history there were more or less than 2 major parties, but it always ended up stabilizing after an election cycle or two back to 2 parties.

What you are asking about is a situation where there effectively becomes only one party. This actually happened in the mid 1810's when the Federalist Party collapsed, and the early 1850's when the Whigs did likewise. What has always happened is that a new second party naturally forms (actually, usually several, but the strongest 2 parties in the system end up surviving).

My favorite way of explaining the US system to those used to Parliamentary Democracy, is to think of US parties as coalitions, not parliamentary parties. What in a Parliament would be a "party" in the USA is a "wing", and a wing is not much more tied to a specific party than a Parliamentary "party" is tied to whatever coalition it may be attached to.

Looking at it that way, in the US the coalitions are formed before the elections (via party alignment), and then the voters get to choose which of the two is the ruling coalition, and which is in opposition.

Walter Karp argued that US political parties form a virtual bipartisan oligarchy which colludes together for the primary purposes of mutual self-preservation and maintaining organizational power in his 1973 Indispensable Enemies: The Politics of Misrule in America.

A few quotes from Indispensible Enemies off a website put up by a Karp enthusiast, theorizing that since the Civil War political party collusion is why state districting has proven surprisingly resistant against sweeping change, despite many other substantial social changes having occurred:

p33

State party bastions are one consequence of two-party collusion, a collusion so tight in many states, and in almost all the large ones, that the two party organizations actually form a single ruling oligarchy. These bastions, however, are not arbitrary divisions. They are, for the most part, districts whose inhabitants did strongly support one party or the other at a much earlier time in our history. As party organizations gained control of their parties, their mutual cooperation simply froze the earlier pattern of partisanship. Each party organization ceased to compete seriously where the other party had been strong, for only through mutual cessation of electoral competition can party organizations maintain themselves and so retain their power. This is the reason for "the long persistence of county patterns of party affiliation" in so many states, to quote an essay by V. 0. Key, Jr., and Frank Munger in Democracy in the Fifty States. It is also the reason these partisan patterns often reflect Civil War party divisions: it was in the decades immediately after the war that bipartisan machine politics began taking hold in one state after another.

p34

Given the motives of state party organizations, it does not matter whether two districts have become virtually alike since the Civil War. Under two-party collusion, a district is permanently marked off as "Republican" or "Democratic," and the voters in these districts can only follow suit, which means, simply, that the majority of voters will not often support candidates who are put up to lose.

They did when they had a common external enemy, and the world was better for it.

The United States fought the fascists during World War II (1941-1945), and the Soviet Union during the Cold War (1945-1991). During this period, the country faced a common external enemy, and Democrats and Republicans cooperated to unprecedented levels. Among the accomplishments:

• Willingness from both political parties to engage in foreign affairs, form alliances, and negotiate treaties and trade pacts.


• Serving as a political and economic superpower.


• Actively participate in the United Nations and NATO.


• Aiding other countries through the International Monetary Fund and World Bank, which helped the reconstruction of much of Europe and Japan after WWII.


• Development of the Interstate Highway system.


• Funding (or in some cases, directly pursuing) scientific research through the National Science Foundation, National Institutes of Health, Department of Energy, and DARPA.


• Through DARPA, funding the initial development of the Internet that you are now using.


• Providing funding for public elementary, secondary, and higher education.


• Providing stable government institutions. As much as Reagan talked about cutting government, in practice most agencies grew during his tenure.


• Enthusiastically operating a space program -- and most notably -- putting men on the moon.

After the fall of the Soviet Union, the enemy became each other. Democrats and Republicans turned on each other, in increasing rounds of tit-for-tat. Indeed, during the last 25 years we have seen the following:

• Paranoia that the United Nations is a world government which is controlling American politics.


• Using foreign countries as a scapegoat for job losses.


• Isolationism.


• Withdrawal from NATO, NAFTA, TPP, and other alliances and trade pacts.


• The loss of international respect and influence of the United States.


• Cutting foreign aid (particularly for "s---hole countries").


• Deteriorating infrastructure (e.g. collapsed roads and bridges, contaminated public water supplies). Refusing to repair or replace it because of a fear of taxes and government spending.


• Cutting scientific funding (e.g. Superconducting Supercollider).


• Mocking science as an "opinion" (e.g. evolution, climate change, endangered species).


• Refusing to regulate the monopolies that control much of the Internet.


• Cutting funding for public elementary, secondary, and higher education (mostly by state governments).


• Turning education over to for-profit corporations.


• Mocking (e.g. "Deep State"), defunding, and harassing government institutions.


• Allowing the space program to wither to the point where we use Russian rockets to send astronauts into space.

The Democratic and Republican parties colluded to disenfranchise 3rd parties by creating the commission for presidential debates in 1987.

The Wikipedia page goes into excellent detail on the history of this commission - I'll include several tidbits of information from it:

In 1988, the League of Women Voters withdrew its sponsorship of the presidential debates after the George H. W. Bush and Michael Dukakis campaigns secretly agreed to a "memorandum of understanding" that would decide which candidates could participate in the debates, which individuals would be panelists (and therefore able to ask questions), and the height of the lecterns. The League rejected the demands and released a statement saying that it was withdrawing support for the debates because "the demands of the two campaign organizations would perpetrate a fraud on the American voter."

Ross Perot ran for President and was able to participate in the debates. He garnered a small but significant amount of votes in 1992.

In part due to Ross Perot's and Ralph Nader's impact in the 1992 and 1996 elections, the commission (run by Republicans and Democrats) in 2000 changed the rules for who could participate in the debates which, for all practical purposes, eliminated the chances for a 3rd party to get into the debates.

In 2000, the CPD established a rule that for a candidate to be included in the national debates he or she must garner at least 15% support across five national polls. This rule has been controversial as it has effectively excluded U.S. parties other than the two major parties.

This rule has successfully been challenged in court due to the 15% being arbitrary.

In February 2017 the suits by Johnson, Stein et al were reheard and the judge ruled that the Federal Election Commission had not provided sufficient justification for its decision not to engage in rulemaking, and ordered the Commission to either provide a more sufficient justification for its position, or to alter the Commission's rules.

The Republicans and Democrats colluded to exclude practical challenges to their hold over the political system.

let's turn the question around and assume that we already have such a situation where both parties have (at least de-facto) merged into a single Party. Then because the public opinion on many issues tends to have a 50-50 split due to social dynamics, this will cause tensions within the Party. Many issues will have a typical conservative or liberal points of view, so there will be strong correlations between Party members and their views; typically two party members will agree with each other on most issues or they will disagree with each other on most issues. This will then ultimately cause the Party to break-up into a liberal faction and a conservative faction.

This instability of a single Party state will then prevent two dominant parties from merging into a single Party.

The primary driving force for U.S. members of congress (MoC) is pleasing your constituents and pleasing your donors; with pleasing your party's national convention being a third priority. Despite many of the other answers, an American MoC doesn't vote in an attempt to preserve a uniparty. If a MoC represents a district heavily predicated on sugar, and they receive large donations from the oil lobby, then that MoC's voting record is going to reflect a lot of pro-sugar, pro-oil bent. They might vote nay on anything outside of pro-sugar, pro-oil interests unless they can get a small pro-sugar or pro-oil earmark added to the bill or curry a favor that they can leverage later. There are a lot of MoC who do not have enough spine to vote in a way that would be politically unpopular in their district; i.e. pleasing the district voters is paramount. This is mostly out of self-interest; but anyone who believes in Republican representation respects the importance of representing your constituency's values, tempered with your own greater judgment (you were selected to be the smartest version of your constituency, after all).

However, many politicians actually do have ideology, and they got into politics for a reason. Outside of the tactics they use to get elected and stay elected, they still have an ambition to set policy on some economic and/or social issues that they care about. This is the battleground of most congressional votes. Most bills connect with each MoC in some ideological way, either for or against, but fail to connect with the special interests that they are personally subject to. Apart from the MoC's ideology, each bill tends to have some implication to the party platforms, and so all other things being equal, a MoC is likely to vote in according with their party platform. For instance, the DNC's platform favors greater funding for public education, so if a bill came up that appropriated greater funding for public education, it is likely that almost all Democratic MoC would vote in favor of it, excluding possibly 1 or 2 who might not be able to due to their district's special interests.

There are very few times or motivations when it mutually benefits both parties to collude to pick election winners. Party polarization actually drives greater campaign donations and richer politicians, so collusion would generally not benefit them on the whole. Moderation would occur either as a tactic to seize moderate voters and win elections, or out of the altruistic recognition that moderate policies are the most tolerable for the greatest amount of the population.

There are, however, a small number of policies in which the two parties do benefit from cooperating on. For instance, any radical campaign finance reform would strengthen independent parties, weaken Democrats, and weaken Republicans even more. Also, replacing the first-past-the-post (FPTP) election system with a superior system would strengthen independent parties while hurting the dominance of the top two parties. More inclusive public debates would strengthen independent parties while hurting the dominance of the two top parties (they would also reduce the public's ability to decide between the Republican and Democratic candidates, which is actually problematic, given that some voters already struggle to pick the better candidate). These are all areas in which the top two parties are unlikely to vote for reform.

But anyways, to the original question: why WOULDN'T they collude? The answer is because there is nothing stopping an outsider like me from running in a party primary, trying to get the party nomination, possibly winning it, and then trying to win the general. Even if the RNC offered to concede a loss in Ohio in exchange for a win in Illinois; they can't stop me, a self-interested party, from trying to run and pursue my own interests, which are in direct conflict with their agreement. Another reason is because there is genuine animus between the parties. A third reason is because there are not any real amount of benefits to this collusion -- why compromise when you can attempt to win it all? A fourth reason is the risk of facing charges of election tampering or conspiracy to defraud the public if they are caught. I honestly don't see how a rational mind could see there being more benefits than liabilities to party collusion (at least in America's current political environment. I could definitely see it in a system where several strong parties are all legitimately in-play).

There are two possible conspiracies here, and I will address them both:

First, you could have an open or mostly open collusion. This would take away the illusion of choice from the population and likely lead to a countermovement, which within a short time would set up a third party and gather a considerable amount of votes, enough to upset the balance of power. The reason is that the feeling of control is a core psychological human need. Helplessness is a terrifying feeling (and is, in fact, a core element of any effective terrorism strategy). If you want to prevent a third party demolishing your conspiracy, you need to subvert the election process to the point of abandoning it, which is likely to provoke an even stronger reaction from the people. Such takeovers have been successful a few times in history (Nazi Germany being the most well-known example), but they required extraordinary external circumstances to let the population accept it and not rise up in revolt.

Second, you can have a hidden collusion which maintains the illusion of choice, but ensures through backroom dealings that on important matters, a consensus is maintained. Conspiracy theorists claim that this is actually the status quo. There definitely is some collusion on matters where incidentally both parties have a shared interest - for example, election reforms are unlikely to pass despite making rational sense because both parties benefit from the system as it is now.

There is a third alternative that maintains that political parties are a scam anyway and real politics is made elsewhere. There are in fact a lot of real-world incidents were politics were made by the party you would least expect it from (e.g. the decidedly unsocial reform of the unemployment system in Germany by the social-democratic party in the early 2000s), and there are numerous people in second-rank political positions (e.g. directly underneath ministers) who have been in that position during multiple administrations with opposing agendas, during which the ministries politics did not actually change very much, indicating that they depend more on its top-level leadership than the minister heading it.

So all of that put together leads to the conclusion that the current system benefits more people better and carries less risk of being overthrown than an outright takeover by an open collusion. If the actors in the system act rationally, they will maintain and evolve the current system over any kind of powerplay into the direction of tyranny.

tl;dr: Because it works

It depends on what you mean by 'collusion'. In a very practical sense, there's no need for 'collusion' in the "conspiracy theory" sense, since the same outcome can be achieved by political means, namely promoting voting systems which effectively impose a penalty to any voter that chooses to vote outside of a two-party system, and secondly using the strong position of these two dominant parties to dictate what constitutes acceptable political discourse and to what extent, effectively drowning out debate which could come from, or benefit other sources. (what Noam Chomsky refers to as "severely limiting the spectrum of acceptable opinion, but allowing very lively discussion within that spectrum", which he claims is one of the main methods in use by governments to keep the voting public unwittingly, yet effectively, "passive and obedient").

But are the two main dominant parties in such bipartisan systems aware of the joint monopoly effect and how to maintain such a status quo? You bet. Does this make it "collusion"? Probably not in the strict sense. What it does make it is a bad political system, one which neither of the two big parties will have any real incentive of changing.

A good example of this is the recent (2010) Alternative Vote referendum in the UK, the purpose of which was to switch from a 'first-past-the-post' voting system to a 'ranked roting' system, which had a very good chance of enabling broader and better representation of public opinion in parliament, at the expense of an ostensibly "strong", rigid, purely bipartisan system. Both major parties cunningly handled the referendum, not by exploring whether the change would enable better representation and choice for the average voter, but by limiting public discussion in the run up to the referendum to the debate of "which existing parties 'benefit' the most from such a change" (effectively rephrasing the problem from one of voter freedom to one of party preference).

The outcome of these lively debates was then used as an argument to persuade voters to vote accordingly in a 'partisan' manner, and, unsurprisingly, the two dominant parties favoured no change to the status quo. Therefore, rather unsurprisingly, the referendum overwhelmingly voted for no change to the first-past-the-post system, despite the obvious personal benefit and increased freedom of choice the alternative vote would have afforded to individual voters at a personal level. It was subsequently shown that most people who voted didn't really get what a 'ranked' system was, or why it mattered, they more or less just voted in the same way as they had done for the parliamentary elections, based on which party they supported and what that party had asked them to do.