Hypothetically, what if the choice in an election comes down to two candidates--one who supports abortion policies, and the other who supports an unjust war? Whom would--whom should--you choose?
Commenter "GloriousRuin" passes along this imaginative dialogue by political philosopher J. Budziszewski that I think is quite helpful. There's probably no better way to get a quick introduction to the distinction between formal and material cooperation than this dialogue!
If I may be permitted one additional comment: I don't like the label "single-issue voter" and don't use it to describe my position. I'm not sure there's a better term--but the fact remains that there are many issues that are of concern to me. By using the term "single issue," I think we suggest that an issue like abortion is the only issue of serious consequence in this election. Some may believe that, but I don't.
Nevertheless, the fact remains that abortion is the great moral outrage of our day--and no one, for example, in the mainstream media (or in many churches) makes a peep about the daily slaughter of the weakest members of the human race.
The following video is very painful to watch--but I believe it is necessary as a reminder that we are not discussing abstract policies and arguments but rather innocent lives being systematically killed: