If my choice is between one guy who’s for abortion vs. another guy who’s for abortion, I will pick the least damaging of the two. If I vote 3rd party, that would be like voting for one of the abortionists anyway, because I take a vote away from the least damaging one, and I might as well have voted for the more damaging.
Because our entire political system is based on 2 party structures, that’s how it works on national level elections.
By choosing not to vote, I let my vote against the worst candidate not matter.
By choosing to vote for a candidate who has no change of winning at all, I let my vote against the worst candidate not matter.
I cannot do that.