Do you mean DNC or the voters? DNC would "support" him in the general election, I mean they have no choice if he's the nominee. But the electorate are terrible lemmings, particularly in these multi-billion-dollar elections in the US. If Republicans or Russians or whoever can run a few successful anti-Bernie ads it could be enough for 5-10% of laziest Democrat voters to see it as an excuse to stay home and hand the win to Trump.

I doubt there is some major Democrat establishment conspiracy, I don't think there was one in 2016, there's just lots and lots of stupidity and incompetence and hurt pride and laziness.