"Donald Trump's reprehensible call to bar Muslim immigrants from entering the United States tracks an exam question I’ve been giving my immigration law students since Sept. 11."

"Would such a proposal be constitutional? The answer is not what you might think..."

Unless you've read all the many other op-eds dotted about mainstream media, but this one, by Temple lawprof Peter J. Spiro, caught my eye because I need to write a couple of law school exams and I have never — in 30+ years — reused a question. I've never even taken the same question and tweaked it with the aim of convincing myself I wasn't reusing it.

Anyway. Professor Spiro does a fine job of explaining the Supreme Court's "extreme deference" to Congress and the President under what's known as the "plenary power" doctrine:

It dates back to the 1889 decision in the Chinese Exclusion case, in which the court upheld the exclusion of Chinese laborers based on their nationality... More recent decisions have upheld discrimination against immigrants based on gender and illegitimacy that would never have survived equal protection scrutiny in the domestic context. Likewise, courts have rejected the assertion of First Amendment free speech protections by noncitizens. Nor has the Supreme Court ever struck down an immigration classification, even ones based on race. As late as 1965, a federal appeals court upheld a measure that counted a Brazilian citizen of Japanese descent as Asian for the purposes of immigration quotas. In the context of noncitizens seeking initial entry into the United States, due process protections don’t apply, either....

The courts have justified this constitutional exceptionalism on the grounds that immigration law implicates foreign relations and national security — even in the absence of a specific, plausible foreign policy rationale. The 1977 Fiallo case, for instance, involved a father seeking the admission of his out-of-wedlock son from the French West Indies — hardly the stuff of national interest.

Indeed, contrary to the conventional understanding, President Trump could implement the scheme on his own, without Congress’s approval. The Immigration and Nationality Act gives the president the authority to suspend the entry of “any class of aliens” on his finding that their entry would be “detrimental to the interests of the United States.”

Spiro goes on to say that the Constitution has meaning beyond what the courts are willing to detect and enforce. As he puts it: "Clear popular consensus can establish constitutional norms, with or without the courts."

Mr. Trump’s plan has triggered an uproar across the partisan divide. Perhaps a religion-based immigration bar may be consistent with court-made doctrine. But it doesn’t reflect our deeper, broadly assimilated understandings of the Constitution.

But is there clear popular consensus? I guess we should at least wait for the next round of polls, because Trump is immensely popular, at least with a sizable enough sector of Americans to wreck the clarity of the picture of popular consensus. Politicians and commentators are trying to lead a chorus of shaming, but Trump may be saying what many are thinking but don't want to be caught singing out.

This "constitutional norms" idea is self-canceling. Trump can't do what he's proposing unless he gets elected, and he won't be elected unless people support him, and if people support him then those constitutional norms that forbid what he's proposing don't exist.