"And is it actually Catholic doctrine that, in a situation of two evils, one should strive to do the least harm possible? "
Then catholics are wrong. The correct action should be to deny them both with extreme prejudice.
It is possible as a Catholic to make such a moral decision defensibly and deny both if they are equally wrong on the questions of life. But it is permissible to choose between two candidates so that the one who will do the greatest amount of harm does not benefit from one's protest.