“The less trust you have in the insiders, the more likely you are to turn to an outsider,” Mr. Popkin says. “The less you trust your doctor, the more likely you are to get a second opinion.” Mr. Trump’s second opinion confirmed what voters wanted to believe: that entitlements could be fixed without cutting benefits and that slapping tariffs on China and companies that move jobs overseas could revive growth.

Even though voters use logic to make inferences, Mr. Popkin underscores that “reasoning and rational don’t mean right,” and that “passion can overrule the fine print.” For example, “If you love somebody enough, you’ll ignore the problem they’ve never had a job or they’re not divorced yet.”…

If it’s any consolation, Mr. Popkin doesn’t believe that Mr. Trump will permanently destroy the Republican Party. “He got enough votes to take over the party for a year,” he says. “I think it will pass.” From time to time, parties indulge extreme candidates, which Mr. Popkins considers unhealthy because it “makes it harder for the other side to be reasonable.”

He estimates the chances of Mr. Trump’s defeating Hillary Clinton at 1 in 6—better than Mr. Popkin would give to Mr. Cruz if he had won the nomination. “Name a state that Cruz would win that Trump would lose,” he says, while it’s possible, if unlikely, that Mr. Trump could win states such as Pennsylvania and Wisconsin that wouldn’t have gone for Mr. Cruz.