You do raise the ultimate question:
When is it grave matter to vote for a particular candidate?
Is it never sinful, because all candidates are sinners, and promote deranged policies?
Or, is it a weighing: One candidate promotes 6 abominations, while the other only promotes 2-3 abominations (as Matthew pointed out earlier)?
Or, does one abomination per candidate rule them both out?
And as regards the non-Catholic "lesser of two evils" schtick, since there is no necessity/compulsion to vote, how will one who votes for evil claim they had no choice?
Keeping in mind that the classic example of "double effect" is one man tied to the railroad track, while a bus of children stalls on another railroad track, and the person must make a choice to save one of the other, but cannot save both. In choosing one, he does not will the death of the other. But I do not see this necessity to choose present in the context of political elections (or, at least not between these two faulty candidates).
In other words, it is not clear to me that double effect is relevant here.
But the "lesser of two evils" without necessity is even on shakier ground: It is a simple matter of overlooking all the abominations of one candidate because they support their particular interest, while condemning the abominations in the other candidate.
It seems hypocritical and immoral somehow to me.
I think the underlying philosophical question is, what is a vote? Is it an absolute endorsement of the candidate? Or is it a strategic calculation, much like the wielding of a weapon?
In neither case I do I think we should always vote for the "lesser evil" because if we do that we just get more terrible candidates. So either way we have to have standards.
However, if a vote is really a full throated endorsement, than we can't really vote for anyone. We couldn't really even vote for Ron Paul (and I will admit, I think it would've been foolish not to, given the possibility) because he did *on principle* believe things like sodomy and the morning after pill should be legal (though he would have absolutely allowed any state to disagree with him) and that's objectively wrong, both should be banned.
On the other hand, if a vote is a strategic calculation than we make a decision, prudentially, OK will voting for this candidate *actually* help to promote Catholic priniciple in this country moreso than not doing so. And if we think it will, we should vote. If we don't, we shouldn't.
I don't think third party candidates are (usually) viable options in the sense of actually being able to win elections. The way the system is set up precludes this. However, they certainly can signal preferences. Voting for the Constitution Party candidate over Mitt Romney or Donald Trump or whoever essentially signals "you lost a vote because you weren't conservative enough."
So, if I'm right that a vote is a strategic allocation of resources rather than a full throated endorsement, one could still argue against voting for Trump on prudential grounds, and they could be right.
But unless the Church has clearly ruled on what a vote is (and as far as I'm aware it hasn't), I don't see how we can accuse of mortal sin, or even sin at all, *unless* a Catholic makes whatever choice they make for a non Catholic reason.
I'd be inclined to even extend this to a vote for Joe Biden *if* the person did it for an actually Catholic reason, I just don't see how they possibly could do so *unless* the argument was something purely strategic and having zero to do with character. Something like "I'm gonna support Biden because a lot of liberals hate him and he'll make a fool of himself and of liberalism." I *think* that would theoretically be acceptable, whether its truly prudent or not.
However, I can think of more possible reasons to get behind Trump. One obvious one is that, while Trump does support allowing evils like sodomy, he's probably doing this stuff to get elected. I see no evidence that Trump actually thinks that Christians who disagree with sodomy are bigoted or should be shut down or anything like that. But we know Biden does think that. Trump also seems somewhat woke on the dangers of Covidism, even suggesting rhetorically that he might force governors to allow churches to reopen (which he can't do constitutionally but nonetheless.)
One could also make compelling reasons not to vote for Trump, but I don't see how one could plausibly argue that he's the same as Biden.
But again, unless the Church has given something clearer that I'm not aware of, I don't see how we can say *either* choice is a sin. this seems like a prudence issue.