President Biden’s campaign is working to reach across the generation gap to the tens of millions of predominantly younger voters on TikTok‚ where the challenges are daunting and the rewards difficult to track. The obstacles range from anger over the war in Gaza to what social media experts describe as the unavoidably uncool nature of supporting the administration in power. Mr. Biden‚ 81‚ joined the app owned by a Chinese company last month‚ in what was widely seen as an effort...