Speaking purely as an electoral issue, Biden and Harris fucked up on Israel/Palestine after October 7. They did nothing to keep voters who felt that was a key issue on side. Now, there's a lot of dipshit gen Z "leftists" who post on social media about how it's because Dems love genocide, which is stupid as hell. That's not Fantana, though.
Dems fucked this up for the same reason they fuck everything up: they've got no actual understanding of voters so love to commission a hundred focus groups and look at a thousand polls before figuring out their political stance on anything. They're indecisive pussies who love paralysis by analysis. And they drastically misunderstood Bibi and the modern Israeli state. So Biden spent an entire year letting Bibi publicly embarrass him repeatedly while the State Department kept scrambling to pretend that it wasn't as bad as the entire world could see it obviously was.
I don't even necessarily think Biden stopping weapon sales once it became clear Israel didn't give a fuck how many civilians and children it killed would've swung the election; it could've turned off just as many voters who barely voted D (if not more) than they turned away with their cowardice. But what they did was consistent with the image much of the public has of Democrats as feckless pussies who won't fight for anything. And that's an image they've earned through incredibly hard and consistent efforts at being nearly the biggest pussies they could be for several decades.
I can and do blame the voters. I can and do blame the Democrats who fucked it up too. There's no shortage of blame to go around. But if we're lucky enough to still have fair elections in 2 years, we'll both need to persuade a lot of voters to stop treating voting like they're fucking consumers who only need to vote for a candidate if they really really like them, and get Democratic officials and candidates to realize that they've got to deal with the voters who actually exist and not mythological voters they dream up in their heads who might as well be aliens for all they resemble actual people.