He really can’t drop out. He could be replaced but the legal maze of local and state deadlines and printed paper ballots that would have to be destroyed would abridge on early voting rights.
Rfk is having that problem right now in a few states. https://slate.com/news-and-politics/2024/09/north-carolina-robert-kennedy-early-voting-trump-sabotage.html
In battleground states the win/lose margin will quite possibly come down to a few thousand votes, particularly in PA. The endorsement might not change anyone’s mind, but to her millions of fans it raises the elections’ importance for anyone who doesn’t “care about politics”. It turns a civic issue into a social group issue, and people care more about the latter. It’ll pull people reluctant to go to the polls to show up and vote.