TikTok dropped a public hammer on Washington on Thursday‚ sending a direly worded alert to users urging them to call Congress and stop a bill designed to cut China’s ties to the app. It signaled how seriously the video-sharing giant is taking Congress’ latest effort to rein in a platform that has long been under fire from Washington with little regulation to show so far. The move recruited TikTok’s vast user base as a new kind of influence weapon — a maneuver that some tech companies have...