Here's the thing...you put a gun in my hand and tell me I have to murder one or a hundred...I'm not going to do either.
Not really the parallel though. Neither is the one and one hundred you follow it with. I'm not talking about you committing a crime. I'm not talking about anyone committing a crime. I'm illustrating that the lesser of an evil can still be a comparative good, that inaction can work a greater harm than action, depending.
There is a God and it ain't me. I don't have to accept that I must do evil so that a greater evil won't come to pass.
Again, you've added that new element, of you doing evil, to make the choice. But that's not the choice. Moreover, I'd argue that if you understand your inaction might do more harm than action, allowing the greater harm might itself be considered an evil act, a sin of omission, failing to save those you could have for the sake of a principle which should be fashioned to serve the good.
So I don't have to choose between two evils because there is a God.
I'd say you do. A failure to forestall is a choice that empowers. That is to say, not voting is the same as voting for the stronger candidate. Now if the stronger candidate is the greater evil you endorse him de facto. That you turn to the crowd and declare your hands washed is theater if you understand that.
I don't require perfection from a political candidate, else I wouldn't vote for anyone. But I'm comfortable with some bare minimum standards such as, for example, not winking at the murder of the unborn.
It's not an issue the President will decide. It's a thing we'll end the way slavery was ended across the pond, when a strong majority understand the immoral nature of the institution then we will amend our law to illustrate that understanding.
So maybe it is the underlying principle. But that principle is the good here, when all you have to offer are evil choices.
I'd say that's the strong argument from principle. And I've given you the problem from the practical side of it. It's not an easy thing no matter where you come down.