When Donald J. Trump called on Monday for “a total and complete shutdown of Muslims entering the United States,” many legal scholars were aghast and said that such a ban would certainly be struck down by courts as blatantly unconstitutional. David A. Martin, a University of Virginia law professor who was deputy general counsel to the Department of Homeland Security in 2009 and 2010, noted that as recently as last May, in Kerry v. Din, the court reaffirmed its commitment to that standard in a 5-to-4 ruling. It dismissed a lawsuit by an American citizen who challenged the government&...