Last week, the U.K. Parliament had second thoughts about its six-day-old TikTok account, shutting it down after Conservative MPs — including three who have been sanctioned by China — wrote a letter to the Speaker warning that the app posed an unacceptable security threat. “The prospect of Xi Jinping’s government having access to personal data on our children’s phones ought to be a cause for major concern,” the MPs wrote. In the United States, with crucial mid-terms ahead, Democratic lawmakers are taking the opposite view — they are focused on leveraging the app to rally the Gen Z voters they...