But even after that change, the court adopted an extraordinarily solicitous standard: So long as the government cites a reason that sounds legitimate on its face for barring a foreigner from entering the country, the justices will uphold it without further scrutiny.

In practice, since establishing that standard, the Supreme Court has never overturned a decision to deny entry to someone.

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’s decision to deny a visa to her husband, an Afghan. The government said it had denied the visa because of unspecified terrorism concerns, and the majority of court said that was good enough without further scrutiny.

Still, the specialists said, Mr. Trump’s proposal to block the entrance of over a billion Muslims on the basis of their religion is so extreme that it was possible a Supreme Court majority might be willing to strike it down despite its very deferential standard for scrutinizing such policies.