I would ask the Democrats here just how they would have felt if in 1976 the Democrats nominated a Democrat who, explicitly, refused to endorse George McGovern in 1972.
Probably similar to how they'd feel if the Democrats in 1976 had nominated someone who'd said that LBJ should have been impeached.
Exactly. Because endorsing McGovern or Mondale is one's duty to the party. The "Republican = Trump" argument that dems are making is very hypocritical because dem leaders would expect everyone in their party to endorse someone obviously terrible as well.
If the Democrats nominated someone as bad as Trump, and the Republican candidate was better, I'd certainly want people to avoid endorsing that person.
YOU would, yes, I believe that, but I do not believe for a second that party leaders like Pelosi and Schumer would endorse a republican in that situation. It would be all "xxx scandal" or "SUPREME COURT" or "Fifth Bush Term!". Congress Critters would be forced to endorse the nominee, and punished if they steadfastly refused. Same thing is happening now with the GOP.
Pelosi and Schumer wouldn't, no. But if someone who didn't endorse that person wanted to run for President in the next cycle as a Democrat, I'd want them to have a fair chance. Let the voters decide.
Similarly, if in 2020 someone who had enthusiastically endorsed Republicans in every election as long as they'd been alive decided to run in the Democratic primary, I'd be cool with letting them do so. If the voters don't want that person, then they won't vote for them.