AOC already does everything in her power to undermine Biden, do you really think she supports him? She'll reluctantly endorse him, as is her duty, and then continue undermining him and making demands of him every step of the way.
It was Joe Manchin, specifically. But his vote wouldn't matter if we could just win one more Senate seat, which would be a lot easier if we weren't being kneecapped by progressives (especially the squad) at every opportunity.
