As the summer of Donald Trump came to its end — and the prospect of a springtime for Trump no longer seemed like a gag — the quest to explain the billionaire’s runaway clown car went into overdrive. How could a crass, bigoted bully with a narcissistic-personality disorder and policy views bordering on gibberish “defy political gravity,” dominate the national stage, make monkeys out of pundits and pollsters, and pose an existential threat to one of America’s two major parties?.....