Just want to add that although Harris ate shit in the primary, she (anecdotally) appears to be much more popular than Biden among virtue-signaling liberals. Her organization was also pretty hopeless in the primary, but she could do better in 2024 if Biden doesn't run.

I am not really a fan of AOC anymore but I think she has a decent shot of taking the nomination if she runs for president on M4A and a GND. Libs would have trouble criticizing her because she's a woman of color. Although she's alienated a lot of her former supporters (like me) with her support of Biden and imperialism, she likewise should not be underestimated. For all her faults her victory over Crowley was pretty amazing, and her organizational abilities seem to be at least as good as Bernie's if not better. All libs could do in response is whine about AOC Bros and secretly write in "Adolf Hitler" when Harris drops out of the primary before a single vote is cast and AOC is the only choice left. And although AOC drives Republicans absolutely nuts, I also think she could beat whoever they throw at her.

That being said, left challenges to Democratic incumbents have historically gone absolutely nowhere since Truman if I'm not mistaken.