The new socialist movement believes it can remake America—but its growing radicalism may prove the very thing that drives voters away and fuels a counterrevolution. Win some blue-state and blue-city races, and the cocky new socialist Jacobins believe that they have either already taken over the Democratic Party or will soon absorb it. And in reaction to these new swarms, an increasingly terrified and ossified old Democrat guard either limps away from the hive or invites them in to take over more. It is fascinating but ultimately depressing to watch old-style Democrats say or do anything to avoid the new...