Biden and the US Congress are absolutely responsible, Israel couldn't have gone on this long or killed as many people without US support.
Kamala Harris could've created daylight between herself and Biden after she got nominated, but she didn't. Her lip service to innocent lives destroyed in Gaza and Lebanon has been pretty weak. I don't think it's likely she'd be much different than Biden. Maybe the political calculation (not that I necessarily agree) is that Harris has little to gain and a lot to lose by staking out a position on a conflict that may no longer be happening by the time she would take office.
Had Harris spoken out against US support, she'd have been seen as undermining Biden and US policy, as well as a politically potent US ally during the heat of the campaign. That's not a good enough reason not to for me, but it might be for Harris -- assuming she disagrees at all with US support of Israel's ongoing campaign of ethnic cleansing and genocide, which she may not.