She distanced herself when Biden was starting to really commit to the genocide denial, which is probably enough for a lot of people. Biden is pretty extreme, even among democrats who support Israel, and he’s been the one driving the wagon. Harris doesn’t have the same baggage or history of fantacism when it comes to Israel. She shows more awareness of the generational sea change regarding attitudes towards Israel.

As for the prosecutor thing, I don’t think that’s something in the same scale of problem as the genocide support. People act like because centerists/“leftists” won’t compromise on genocide that means they’re unwilling to compromise on everything but that’s not true. Such flagrant disregard for basic humanity is a hard limit for many people though. People understand the stakes, Biden’s zealotry is just that extreme and alienating. It wouldn’t have taken much for him to avoid this problem, but combined with the faltering mental capacity he’s too politically toxic at this point to recover.

Polls show that people will vote for the lesser evil that is Harris, they won’t vote for the outright evil that is Biden’s fanatical, blind support of genocide. The party leadership sees it, they understand the math is against them regardless of how much AIPAC pays them, this doddering debate performance and politically deaf follow-up was the tipping point.