A number of prominent scholars (including Eugene Volokh, Eric Posner and Peter Spiro) argue that Donald Trump's plan to stop non-citizen Muslims from entering the U.S. is constitutional (though wrongheaded). They may be right as a matter of modern law. But they all rely on the so-called plenary power doctrine, by which the courts have given Congress essentially unlimited power over immigration.
The plenary power doctrine has no basis in the Constitution's original meaning. It was made up by the Supreme Court in the late nineteenth century, at a time that the Court declared an array of powers "inherent in sovereignty" so that the U.S. federal government simply must have them, irrespective of whether they were granted by the Constitution. In short, the Court did not even pretend to find a basis for the doctrine in the Constitution.
Indeed, as discussed here before, it's not even clear that Congress has full power to decide who enters the U.S. — certainly it does not have that power expressly. However, Congress likely does have something close to such a full power as the sum of its other powers — most prominently, Congress could (I assume) use its power over commerce with foreign nations to prohibit anyone operating in foreign commerce from transporting specified people to the United States.
Looking at it this way puts the Trump plan on shaky constitutional ground. The First Amendment provides that "Congress shall make no law … prohibiting the free exercise [of religion]." Denying a benefit specifically to all members of a particular religion seems inconsistent with "free" [unrestricted] exercise; plainly it is under modern law, per Church of Lukumi Babalu Aye v. Hialeah, and I think as well under the original meaning. And no one doubts that the free exercise clause limits what Congress can do under its commerce power. In short, the Trump plan calls on Congress (or the President acting pursuant to a delegation from Congress) to do exactly what the plain text of the First Amendment says Congress cannot do.
Here is the counter. Arguably under the original meaning the rights in the First Amendment (and elsewhere in the Constitution) apply only to U.S. citizens and to non-citizens present in the United States. Andrew Kent (Fordham) explores this broader claim in this outstanding article from 2007: A Textual and Historical Case Against a Global Constitution. In my view this is (or ought to be) the central constitutional question in the debate over the Trump plan: do non-citizens abroad have rights under the U.S. Constitution? If so, the Trump plan seems plainly unconstitutional (as a matter of the original meaning); if not, then Congress indeed has something like a plenary power over immigration, although not for the reasons the Court gave in inventing the plenary power doctrine.
UPDATE: Will Baude writes to point out that a number of important scholars have suggested that Trump's plan might violate the establishment clause. The idea is that the clause forbids the government from favoring a religion, and denying benefits to one religion favors others. If so, and if the establishment clause is understood as a structural provision that limits the federal government generally, rather than (or in addition to) protecting rights, then the fact that those harmed by the Trump plan are aliens outside the U.S. might not matter. By similar reasoning, for example, the declare war clause limits the President's power to initiate war on his own, even if those harmed by the war are aliens overseas. In this way, we might find the Trump plan unconstitutional without having to wrestle with the broader question of constitutional rights abroad.
I'm not at all an expert on the establishment clause, but finding it relevant to Trump's plan strikes me as implausible. As I understand how the 1787-89 framers understood "establishment" of religion, it referred to the creation of a particular government-endorsed and supported Church such as the Church of England and the official religions of some U.S. states, and (I suppose) by extension it referred to the government endorsing or conferring benefits on a particular religion (or perhaps religion generally). Trump's plan does not do this, except in the extremely speculative and indirect way of possibly encouraging some people to convert to Christianity.
Moreover, if one reads the establishment clause to prohibit penalties against a particular religion, that seems not to leave any operation for the free exercise clause. For example, prohibiting Santeria rituals, as in Lukumi Babalu Aye, potentially encourages people to adopt a religion other than Santeria. So (on the reasoning above) this would appear to convert that case, and others like it, into an establishment clause case. That seems a strained way of looking at the text. It seems more structurally sound to read the clauses as complementary: the establishment clause prohibits favoring religion, and the free exercise clause prohibits disfavoring religion.
Posted at 6:32 AM