All you have to do is take a passing glance at Graham Platner, a progressive candidate for U.S. Senate in Maine, to understand why so many Democrats have been frothing at the mouth over his candidacy for months. His beefy tattooed arms and weathered face made him look like a live-action Popeye. He’s often styled in a dirty ball cap and ragged T-shirt, implying a sort of everyman machismo. It’s the kind of look that suggests Platner could be the Democratic Party’s new great white hope — a working-class white man who can speak to class antagonism in an economically...