The nation has struggled to categorize the Democratic presidential candidates. Sen. Elizabeth Warren is some days a “populist,” others a “liberal.” Sen. Bernie Sanders is at pains to define “democratic socialism” as apart from plain, old “socialism.” The media describes Sen. Amy Klobuchar as a “centrist” or “moderate,” even as she insists on “proven progressive.”

There’s an easier taxonomy: Lefties vs. Crazy Lefties. That’s the choice Democrats have in the primaries, and the two pools from which Donald Trump’s opponent will...