This is clearly an area of Catholic theology that has been underdeveloped. Some distinctions are not properly explored.
Let's look at a candidate who's running for county dog catcher. He happens to be Pro Abortion. Is it permitted to vote for such a person because you think he'd be the best dog catcher of all the candidates on the ballot? While I had written about the principle of double effect, voting for this ProAbort dog catcher would have zero practical effect vis-a-vis abortion itself, since this person would not have the authority to further any of his perverse beliefs in his capacity as dog catcher. Yet one of the Catholic theologians cited stated that one can never vote for a candidate of bad principles. One what grounds? Due to scandal? Well, nobody has to know who we voted for.
Let's look at the Presidential race. Let's say that one candidate is Pro Life, the other Pro Abortion. One could very safely conclude that NEITHER of these candidates will do anything practical one way or the other regarding the issue of abortion. So what if on this basis you ignore the abortion issue and vote based on other issues where you conclude that the candidate may actually have some power?
So the interplay between principles and anticipated PRACTICAL effect of one's vote can be rather complex. Catholic theologians need to explore this more in depth.
Then there's "waste your vote" thinking. Let's say that there are three candidates, two from the major parties and a third party candidate. Let's say the third party candidate is a "worthy" candidate but the other two are not. But this third party candidate is polling in the single digits and has no shot to win. Are you obliged to vote for this third party candidate? Or would you say that by "wasting" your vote you would be enabling the less worthy candidate to succeed?
All of this goes back to an understanding of what "voting" actually means. Voting is more than just a pragmatic exercise. I will never vote for an unworthy candidate due to pragmatic thinking. Voting involves an endorsement and in a sense a material empowerment of the candidate. And it all goes back to the nature of "authority". By voting we materially designate the holders of authority, whereas the authority itself formally comes from God. No holder of authority should ever be someone who's positively offensive to God, because in holding authority they represent God in society. Consequently, it's a direct insult to God to vote for such a one, even a Pro Abortion dog catcher or mayor or city councilman.