“…sort of like Bernie losing the Primary but then endorsing Hillary Clinton and then campaiging for her.”
Well, there you have it. The moment that happened, Sanders shot all his credibility.
The polls showing Sanders beating Trump in a heads-up election were always bunk, for two reasons. The first is that pesky electoral college again. There are even fewer states that would have voted for an aging socialist than Hilary won. Trump would have ended up north of 350.
The other is that 100% of the GOP opposition research was spent on Hilary. The moment Sanders took the nomination, all that effort would have immediately been diverted to digging up crap in Sanders’ background, and ever nonsensical socialist utterance he made in his 20’s would have been played back ad infinitum.
A progressive might win a national election, but not a socialist. If Sanders had won the nomination, a #neversanders movement would have booted up within the Dem party itself, consisting of the Dem centrists.