Mr. Trump then headed back over the border, shedding his decorum by the time he got to Phoenix.

In a strident speech given over a steady roar of cheers, he restated his brutally simple message: Criminal aliens were roaming our streets by the millions, killing Americans and stealing our jobs, and he’d kick them all out with a new “deportation force,” build the wall and make America safe again.

The speech was a reverie of immigrant-fearing, police-state bluster, with Mr. Trump gushing about building “an impenetrable, physical, tall, powerful, beautiful southern border wall,” assailing “media elites” and listing his various notions for thwarting evil foreigners. He said the immigration force might deport Hillary Clinton.

By now we should all know better than to take what Mr. Trump says on any given Wednesday as somehow truer than what he said the previous Wednesday, or will say the following Wednesday, and whether what he tells the Mexican president or a crowd in Phoenix is more honest than what he says at a presidential debate or in a campaign ad.

The details may change. Mr. Trump and his surrogates may talk about a real or “virtual” border wall, electronic workplace verification, this or that entry-exit system, an aggressive deportation force or more gradual “attrition through enforcement.” They may talk about legalization someday, years from now, or never. Those talking points ultimately don’t matter — a President Trump wouldn’t have the resources to deport 11 million people. He has no workable plan to seal the border, build a wall or repair the economy once he destroys it by devastating the immigrant work force. He would, however, be able to make millions of immigrants miserable, and break up their families, and damage the country.

To mock him for emptiness is almost too easy. But the fear and loathing that he has tapped into, that so easily won him the nomination, are real. They are real in Arizona, home to one of the nation’s worst state immigration laws, where the political and law-enforcement powers have been arrayed for years against Latino immigrants.