To win a primary, policies must appeal to chronic voters with strong single-issue preferences without turning off enough swing voters to lose the general election. Conservatives have to appeal to gun owners and evangelicals to win the primary, but take care not to alienate socially moderate suburban voters in the process. Liberals have to appeal to teacher unions, global-warming activists, pro-choice feminists and minorities to win the primary, but avoid losing fiscally moderate suburban voters in the general. This mechanical, calculating approach to politics is one reason voters discount political promises and the politicians who make them.

Donald Trump upended all this. He doesn’t carefully tailor his message or stick to tested and approved sound bites. His more straightforward, undisciplined approach elicits a visceral reaction from voters. Mr. Trump’s supporters are famously loyal, and his opponents suffer from Trump Derangement Syndrome. Nobody is ambivalent about the president. He has his base’s trust precisely because he isn’t judicious with his words. While they often cringe at his demeanor, they trust him to say what he thinks…