How can you look at the current support for Trump and conclude you would've won in the US by not supporting Israel?

Because polls before and after the election were crystal clear on this point.

Over 30% of Biden 2020 voters said arming genocide was going to affect their vote.

That's BIDEN VOTERS.

80% of Democrats wanted an arms embargo.

Arming Israel meant giving up millions of votes in swing states, in an election that was lost by extremely slim margins in those states.

And before you ask, it was also clear from polling that ending support to Israel would have cost nearly zero votes from her base.

And the reason the Harris campaign didn't know this is because they didn't want to know. Campaign staffers were instructed to mark anyone who raised Gaza as "no response". Attendees of the DNC conventions were literally plugging their ears and shielding their eyes from protests, or even laughing about them.

Trump won by less than 50% of the vote and there are many polls that show the Biden administration's genocide was massively demotivating to democratic voters.