‘This is not your grandfather’s democratic socialism,” a Washington Post columnist gushes in a video celebrating the congressional primary victory of Alexandria Ocasio-Cortez, a member of the Democratic Socialists of America. As someone involved in democratic socialism in the 1970s, let me confirm that the columnist is right. But not in a positive way.

DSA is a successor group to the Democratic Social Organizing Committee, established in 1973 and led for years by Michael Harrington. I participated in the DSOC founding convention...