Bernie Sanders supporters are already not happy with progressive superstar Senator Elizabeth Warren because she failed to endorse Sanders in the Democratic primary, and they're not likely to be happy with this, either. Asked if she supports the Superdelegate system, Warren said even though she's a superdelegate herself, she doesn't "believe in" them.
She didn't stop there, though, adding a shot that can't be pleasing to fans of a candidate whose only stated path to victory is to convince superdelegates to overturn the will of the pledged delegates, the majority of whom will have been won by Hillary Clinton:
I don't think that superdelegates ought to sway the election.
The natural followup question that Bernie Sanders might ask is "Then what are they for?" That's the rationale behind fighting to get rid of Superdelegates, while also using them just this one last time.
Warren has stayed out of the Democratic primary fight, a fact which Bernie seems to at least grudgingly accept, but Warren's stance here doesn't bode well for him being able to convince any Supers to flip. Anyone willing to cross Hillary isn't likely to also want to cross Warren.