Do you think, that maybe, if Biden had chosen to support another candidate, all those “Blue No Matter Who” types would dog pile people not supporting that candidate?

It depends on how far to the right the candidate is. Get far enough to the left, and they start bein’ like “Party Unity My Ass” and start forming PACs to get Republicans elected

EDIT: I see centrists don’t like being reminded of their proudest moment: trying to get McCain/Palin elected because they didn’t get their first choice in the '08 primaries.