To those who are saying “this happens every election, there’s no reason to panic”:
Wrong.
It is very, very rare for two-thirds of viewers (per the CNN poll) to agree on who won the debate. It is very rare for a candidate to screw up badly in a way that precisely aligns with the opposition’s main point against him. And yes, debates, especially lopsided debates, have historically led to significant and enduring shifts in the polls.
It’s panic time. Somehow, incredibly, we are in a world where the Democrats are probably going to lose this election. And it is by no means obvious to me that sticking with Biden is the better of our bad options. There’s no practical way to replace him as the nominee without his consent, so if he agrees to withdraw, it can be done in a somewhat face-saving “must withdraw due to health condition” way.