A 2013 poll found that most Republicans wanted a congressman who “sticks to their principles no matter what,” over someone who “compromises to get things done.” By contrast, large majorities of Democrats and Independents preferred someone who would bargain with the opposition. Similarly, last October, a poll found that 62 percent of Republicans wanted leaders who refuse to compromise on principle even if it risked a government shutdown, whereas more than three-quarters of Democrats favored cutting deals with the GOP to avoid gridlock.

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.

The proposal to ban Muslims is certainly popular among Republicans: Almost six in ten GOP voters support it. More importantly, the issue establishes an emotional connection with primary voters who feel that Trump is on their side. Emotion is the most powerful force in politics, and it is often far more compelling than arguments based on facts and reason. Once Republican voters identify with Trump, it unleashes what psychologists call “cognitive consistency.” People will rearrange their beliefs to produce an unswervingly positive view of the candidate. Even crystal-clear evidence of Trump’s past liberalism will be ignored or rationalized away. Whereas a single deviation from conservative orthodoxy can sink many GOP candidates, Trump supporters just don’t seem to care about his previous widespread embrace of moderate ideas.