Voting for someone for president who does not oppose abortion is called, in moral theology, “remote material cooperation with evil.” Everyone cooperates with evil at some time or another. It can’t be helped.
Remote material cooperation with evil exists when the person voting, or buying, or selling, etc. does not intend any evil to occur, when the cooperation with the evil is insignificant, and when the evil would occur even if the person who remotely cooperated would have abstained.
Remote material cooperation with evil almost always carries no culpability at all. Should a pharmacist fill a prescription for birth control pills? Does that make him a co-conspirator in abortion? No. What about the man whose cleaning service cleans the office of a doctor who performs abortions in another facility? Is he culpable? No. These people have to work, they have to feed and clothe themselves and probably families, they do not wish the evil to occur. It’s the same with voting. Clinton and Sanders, if one is elected president, might not actively oppose abortion, but they aren’t going to be out rabidly trying to coerce women into getting an abortion. And what they do, they would do with or without my vote or anyone else’s vote.
If you buy your clothes from a company that owns a sweatshop in China, you are committing remote material cooperation with evil. Are you culpable? No. Even if you can get your clothes someplace else? No, still not, however, it would be good if you would get your clothes someplace else. No one, however, can be expected to investigate all the clothing manufacturers out there to make sure they aren’t operating sweatshops in China or India or elsewhere. I have an Internet service provider who employs Indians as underpaid customer service representatives. I keep my Internet service provider even though that is remote material cooperation with evil because if I change, the underpaid workers in India will still be underpaid. I do not wish them to be underpaid, but I have no clout with the Internet service provider, they will go on doing what they do in India no matter what I do, and they are the most reliable one given where I live.
Moral theology is filled with gray areas. We can be culpable when we think we are not, and we aren’t culpable when some think we are. For example, if a man does not want his wife to have an abortion but he drives her to the clinic and pays for the abortion anyway, that is immediate material cooperation with evil, and it carries severe spiritual penalties. If a nurse, who does not believe in abortion takes a job assisting a doctor who performs them because he pays well, that is formal material cooperation with evil, and it carries the same penalty as the doctor incurs.
Gray areas, and even more gray exist in moral theology.