The point of red lines is to not cross them. If you consider supporting a genocide a red line, which is certainly what I was taught, that means the system has moved beyond the pale and you must now take a different approach to politics than the horse race lesser evil logic that you were taught by the ruling class.
Namely, start working on other ways to build political power. The other ways are actually stronger. The ruling class, logically, teaches you to only see politics through a lens that disempowers you. It gives you playdough when you need a knife.
What's really funny, is that this is something both the ancient greeks knew (that voting is nothing more than a popularity contest / theatre piece, since the only people able to fund their campaigns come from the upper classes), and that Marxists in the 1800s rediscovered when they dealt their own theoretical death blows to liberalism.
Upper-class USonians love their reality TV show elections, but most people are smarter than them, and have realized that voting has never improved their lives, or taken war off the menu.
My point is that it doesn't matter who you vote for, whoever it is is going to support genocide. You can choose not to vote. You can vote for someone who has no chance. Or you can choose to select a candidate based on other things they have said or have done.
This is what happens to Democrats. They're idealistic in their youth, but then become cynical and bitter as they see their candidates betray them one after the other.
It is easy, Gideon Levy explains how Israelis do it and I am starting to think it applies to liberals in the US as well. They simply don't see Palestinians as people.