WASHINGTON – The same scene played out at campuses across the country. Charlie Kirk would grab a microphone, take a seat under a canopy — often in busy campus hubs — and invite debate from anyone who came along. His prompt: “Prove me wrong.” Kirk’s open and engaging approach veered from the well-worn tracks of provocateurs who rile audiences in campus lecture halls. It made him a phenomenon, attracting hundreds who crowded around his tent as challengers sparred with one of the nation’s most influential conservatives. It also made him vulnerable. The risks of his exposure became too clear Wednesday,...