it's not that biden has full on dementia, it's that he seems to be noticeably slower, lower energy, more prone to being tongue-tied than he was last time he campaigned 8 years ago
this is a problem for two reasons. one, the media will absolutely pick up that ball and run with it no matter how incoherent/corrupt/insane trump is. two, he's running for a four-year job that starts one year from now, so even the first signs of mental decline are worrisome. old people can get old real fast.