This has been SOP for the Democrats for decades since Reagan-Mondale.
Bashing Indy Leftists for not toeing the Democrat line is their methods of showing their right-wing/conservative wealthy donors they can stick it to the "Left" and still get the latter to vote for them because "Where are they going to go? Republican??"
It's worked so well for so long because "progressives" have so swallowed the bait of "TINA" and sacrificed building an Indy Left movement.