OK fair enough Mr. Lawyer. Look it up and quote for me the existing law that permits closure of the southern border.
I think this that I posted earlier shows that the Constitution pretty much covers it.
Article 4 Section 4
"The United States shall guarantee to every State in this Union a republican form of government, and
shall protect each of them against invasion, and on application of the legislature, or of the executive (when the legislature cannot be convened), against domestic violence."
Article 2, Section 3,
...
he (the president) shall take Care that the Laws be faithfully executed,The federal government has the duty to protect the states from invasion (that is the law)
and the president is charged with executing these laws.(that's the law)
Not sure how it could be more cut and dry.
In addition to this there are immigration laws but I think the Constitution pretty much spells it out. Too bad the government stopped following the Constitution a long time ago.