In the folk wisdom of recent American politics, Donald Trump is a figure of herculean invulnerability to traditional scandal. What lands as a crippling blow to most politicians leaves nary a scratch on Trump, who effortlessly deflected the slings and arrows of the 2016 presidential campaign and paid no discernible price for the “Access Hollywood” tape, his racism or his general incoherence. As the tribune of a discontented, “forgotten” people, this folk wisdom goes, Trump draws strength from...