The other view on the left that drives me up the wall is the idea that every vote from someone ostensibly on the right somehow “contaminates” the candidate

It's ideological purity testing in action.

Lmao you’d think they’d realize that if this was true they should be voting in order to counter-contaminate “You mean politicians tend to be more supportive of the positions of people who reliably vote for them? Have you tried repeating this to yourself in the mirror, slowly?”