I've been discussing,the USCCB guidelines for voting, and their synchronicity/consistency with a few of the papal encyclicals (Evangelium Vitae, for one), with a few priests and several people staunch Catholics...My blog post was getting too long, and rambling anyway, so I decided to answer his question as part 1 and conclude with more cases in part 2. Here is my answer to him (with a couple of minor tweaks).
So, since your FB post says "I vote pro-life first", I put it to you.... How can a Catholic vote for Romney if he is pro-choice in the case of rape/incest.
OK, so here's the thing. As a Catholic is not moral to not vote, because you're abandoning your responsibility to serve society.
On the other hand, unless Jesus or Mary are running for office, you will be voting for a candidate who is involved in some kind of sin. So the question is can you vote for anybody? The answer is yes, you can and furthermore you must vote for the candidate who in your well-formed opinion will do the most good or, barring that, the least harm.
The reasoning behind that is the principle of double effect which says that you can allow some necessary evil to occur in performing an act providing 4 conditions are met.
1. The act itself must be at least morally neutral (voting is a moral good)
2. The good effect of the act must be intended, not the evil one.
3. The good effect is not produced by the evil effect.
4. The good effect is proportionally greater than the evil effect.
So if there are no pro-life candidates you vote for the one who would be the least pro-abortion. Some people would add that even if there is a pro-life candidate, if that candidate has no chance of winning, the best outcome morally is to vote for the least evil candidate who has a chance of winning. That is a prudential decision.
So it could be moral to vote for Romney even though he has exceptions in his pro-life stance if (a) there are no completely pro-life candidates on the ballot or (b) if you believe that the pro-life candidate would not win anyway, and you choose to use your vote to ensure that a worse candidate does not win.
By the way, Catholic Answers has a good Voter's Guide for Serious Catholics that covers these issues and more.
[N.B. part 2 and part 3 can be found at those links, respectively.]