The Democratic Socialists of America conducted an internal membership survey after Sen. Bernie Sanders’s loss to Hillary Clinton in the 2016 presidential primary. Its purpose was to reassess its priorities and expand its electoral coalition. What it found was not far from the caricature of a modern left-wing radical, and a preview of what the rioters and looters sacking major cities across America in 2020 would look like. We’re in the midst of those riots now, and it’s long past time to get acquainted with the extremists who have taken to the streets to turn protests against police violence into violence of their own. According to the Democratic Socialists of America’s own data, nearly a third of its members in 2017 earned over $100,000 a year. Just 6% belonged to a union. A vast majority boasted a college or post-graduate degree. Only 6% of its members identified as “just Democratic Socialist,” with the rest falling into ideological camps ranging from the dull (“progressive”) to the obscenely radical (“anarcho-syndicalism”). The survey provided cheap laughs for those suspicious of the group’s real commitment to “working-class liberation” but also provided a glimpse into how much the composition of the Democratic Socialists of America mirrors the Democratic Party, which its members like to complain is insufficiently left-wing. The two may be more alike than each group cares to admit. Polls from the Michigan primary this year, for example, found...