What does "left" have to do with it? Is it only leftists who object to genocide?
The number of voters dumb enough to switch from Biden to Trump is insignificant. What's going to happen is a whole lot of voters are just going to give up and stay home, and it won't have anything to do with how far left they are.
Just like every damn election, the left is telling the establishment how to attract voters, the establishment is ignoring the left, then the establishment will blame the left. The vast majority of those engaged enough to call themselves leftists are going to vote for Biden. It's the barely engaged working Americans who are going to check out.
Everybody who voted for Biden didn't even vote for Biden. The 2020 election wasn't about Biden or Harris, it was about Trump.