AOC already been on record that she'll support the dem nominee whomever it is months ago. IMO, there's no national Dem that is going to go against the party because they don't want to be seen as a reason why there's no 'unity'.
Indifference/inaction may be another thing, like actually actively campaigning. Bit of a catch 22 since even if Sanders, AOC, or another progressive Dem campaigns hard for Biden, if/when he loses (Donald Trump is teflon, I don't think his supporters care whatever happens, they'll still vote for him), they'll still get blamed by the 'establishment' wing because they need to tell their own donors why it isn't their fault.