One of the great puzzles of the primary season is why a candidate who so recently espoused moderate or progressive views is succeeding in a party that favors purity over pragmatism. Of course, Trump can always claim that he had a road-to-Damascus-style conversion to conservatism. But this claim surely won’t cut it for many movement conservatives. After all, if Trump can flip to the right so recently, perhaps he’ll flop back to the left in due course.

For Trump, the solution has been to announce something so outrageously offensive to liberals, so contrary to every progressive shibboleth, that its utterance immediately disqualifies him from being a leftist. Opposing Obamacare isn’t good enough. After all, some progressives aren’t big fans of the Affordable Care Act. Neither is it sufficient to back gun rights. Many on the left own guns and believe in the Second Amendment. In any case, these are issues on which reasonable people can disagree. What Trump needs is something that is literally unspeakable for a liberal. Trump’s immigration policy is just the ticket. Virtually no progressive would dream of banning the world’s 1.6 billion Muslims from visiting the United States. The very idea represents a kind of exclusionary nativism that is anathema to liberals. Trump’s anti-immigrant language is an efficient way of proving that he has abandoned the left. In a single glorious moment of illiberal demagoguery, he can achieve what would otherwise take months of debate and rebuttal.