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.
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That seems pretty clear cut.
Even so, back during the “Trump’s Muslim Ban” crisis, there were plenty of Leftist legal scholars arguing that this law does not mean what it says it means and that this President has no right to do anything to block anyone at any time. Expect injunctions from lower court judges everywhere - which Trump will this time have to completely ignore...and in doing so create “The Worst Crisis Since Watergate” all wrapped up neatly with a bow for the MSM to talk about non-stop in the days before the election.