Agree or disagree with Bernie Sanders, at least he’s consistent—or so he claims. “Having a long record,” the Vermont senator said in October, “gives people the understanding that these ideas that I’m talking about—they are in my guts. They are in my heart. This is who I am as a human being, and it ain’t gonna change.”

That’s true up to a point. During half a century in public life, Mr. Sanders has dependably denounced “oligarchy” and proclaimed himself a “democratic socialist.” But his definition of the term has radically...