There are just two possible outcomes: Dems or Reps getting power. That's the switch options you have. "Not Voting" simply means letting the trolley take the Rep route and being JUST AS CULPABLE for the results as every single republican voter.
Your fantasy of "not voting" being an actual moral option is like arguing "I disagree with the concept of a trolley, so I'm just going to turn away from the switch". You're morally exactly as culpable, because you made a choice that is morally the equivalent of "not switching tracks".