Phones rise across the room as Chris Rabb steps to the front. It is the second day of the national summit for the Democratic Socialists of America (DSA) in Chicago, and Rabb is fresh off an upset that launched the movement’s summer winning streak, defeating party-backed rivals in a Democratic congressional primary outside Philadelphia. Activists in red shirts move closer to take selfies. Some wear DSA’s rose insignia; others have slogans about ICE or Gaza stretched across their chests. “All right, comrades,” Rabb says, using the group’s preferred form of address with a satisfied grin. He asks whether anyone is...