THE CROWD at the University of California Riverside was riled up. A few thousand people—locals and students at this public university outside Los Angeles—had gathered on a campus lawn to watch Charlie Kirk debate anyone who would challenge him. Many of them knew Mr Kirk, a 31-year-old conservative activist, from social-media videos documenting his debates with college students. These have titles like “45 Minutes of Charlie Kirk SHUTTING DOWN College Kids”. Some have millions of views.