Democrats will continue to act like he's gonna coast into office by virtue of not being Trump. They will continue to ignore all warnings from their left. And they will blame the left when he loses. It's the 2016 playbook all over again.
I guess it depends on what you mean by Democrats. There were some quite high ranked people openly critical and suggesting replacing him (more than I would expect). Dems are not a singular entity… but the only person with the power to replace the candidate at this point in time is Joe Biden himself. So if he doesn’t step down I do expect most high-ranked Dems to fall in line.
If he loses? What's he doing to win? Which group of voters is he trying to pick up? Netanyahu can't vote for him. Republicans won't vote for him no matter how many of their policies he adopts.
Centrist Democrats alone aren't enough. No matter how large a majority they think they command.