Let's say that every single one of them votes for Biden in their respective caucus/primary *but* it becomes clear next summer that Biden is physically unable to run for President.

Do you think a single one of these people would hesitate to say, "Please run Gavin Newsom" in that case, if it was obvious that Trump was going to otherwise win?

If the rules prevent that, then it doesn't matter what we say. Unlike Trump, we have rules and we (usually) follow them.

I'm not familiar with all the rules regarding nomination, but if there is no exception then there is no exception. Period. Hopefully there is something in the rules that allows for a nominee who becomes incapacitated or dies, but maybe there isn't.

I agree with albaby that for the Dems it is Biden or no one. No one else could rev-up a campaign in that short of a time, and the party would be very fractured with little time to get behind the eventual winner.

I was reading, and tend to agree, that the Dems need they need to talk abortion, and stuff like that. Threats to freedoms and autonomy, which the right is big into lately. Throw stuff that Trump says back in his face, but don't focus on him.