I used to really struggle with this one. Becuase in my mind I wasn't endorsing evil by voting lesser evil. I was voting for the difference.

What made me realize and change my mind on the subject was when I realized that for the politican I'm voting for I am NOT endorsing just the things that make them different from the other guy. They actually see it as an endorsement of everything they do. And therefor they see it as a reason to keep doing exactly what they've been doing.

It was me realizing that what I think my vote is for doesn't materially matter.