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President Trump recently tweeted that he may close the country’s southern border if Congress does not appropriate the funds to build a border wall.

... We build a Wall or close the Southern Border ...

^{– @realDonaldTrump (Tweet 1 and Tweet 2)}

So, does the executive branch have the legal authority to either partially or fully close the country’s borders?

The term "close the southern border" is quite broad and has many potential ramifications.

Would such a directive apply to citizens? Legal residents? Commercial vehicles?

Does he mean closing established ports of entry?

Or, when he says "build a wall or close the southern border", does he actually mean the same thing? After all, building a border wall could be interpreted as closing the border.

President Trump is no stranger to broad, sweeping and contradictory terms. For example, he promised repeatedly during his campaign that "Mexico will pay for the wall". (So why is he even asking Congress for funding? That's a whole other question.)

Apparently, payment for the wall is coming, not through a check from the Mexican treasury or deductions in foreign aid, but through improved trade relations negotiated in the United States–Mexico–Canada Agreement (USMCA).

Without the details, most of his "close the southern border" statement is undecipherable.

Considering that the President is focused on illegal immigration, let's assume that "close the border" applies only to non-citizens crossing the border without authorization.

This is interesting because many of these people are already avoiding the established ports of entry. They are crossing wherever they can, regardless of how remote and treacherous the path may be. This is, after all, why the President wants to build a border wall.

So the benefits of closing the border, in terms of curtailing illegal immigration, are not clear.

Legally, the President has the authority to "suspend the entry of all aliens" if he/she finds the entry of aliens "detrimental to the interests of the United States". (The IRS defines "aliens" as any non-citizen.)

United States Code > Title 8 > Chapter 12 > Section 1182(f) "Inadmissable Aliens" reads:

Suspension of entry or imposition of restrictions by President

Whenever the President finds that the entry of any aliens or of any
class of aliens into the United States would be detrimental to the
interests of the United States, he may by proclamation, and for such
period as he shall deem necessary, suspend the entry of all aliens or
any class of aliens as immigrants or nonimmigrants, or impose on the
entry of aliens any restrictions he may deem to be appropriate.

In 2017, the U.S. Supreme Court affirmed this authority in Trump v. Hawaii.

Here's an excerpt from Chief Justice John Roberts' majority opinion:

By its terms, §1182(f) exudes deference to the President in every
clause. It entrusts to the President the decisions whether and when to
suspend entry (“[w]henever [he] finds that the entry” of aliens “would
be detrimental” to the national interest); whose entry to suspend
(“all aliens or any class of aliens”); for how long (“for such period
as he shall deem necessary”); and on what conditions (“any
restrictions he may deem to be appropriate”).

It is therefore unsurprising that we have previously observed that
§1182(f) vests the President with “ample power” to impose entry
restrictions in addition to those elsewhere enumerated in the INA
(finding it “perfectly clear” that the President could “establish a
naval blockade” to prevent illegal migrants from entering the United
States).

The sole prerequisite set forth in §1182(f) is that the President
“find” that the entry of the covered aliens “would be detrimental to
the interests of the United States.”

The power to designate ports of entry belongs either to the president or to subordinates. See for example 19 USC 2:

The President is authorized from time to time, as the exigencies of the service may require, to rearrange, by consolidation or otherwise, the several customs-collection districts and to discontinue ports of entry by abolishing the same or establishing others in their stead.

It follows that the president could close the southern border by removing all the ports of entry on the southern border, leaving no legally designated place to cross.