Once the fringiest of marginal groups, the DSA now has close to 130,000 members, up from 5,000 15 years ago. And its members are winning Democratic primaries across the country.

Megan Romer, national co-chair, said its growth reflected a deep frustration with a system she said was failing young people. “People are, like, well, OK, capitalism’s not working,” she said in an interview. “So what else is there?”