So let's get down to brass tacks. Who can I vote for this November?
The short answer is, I don't know.
A lot of folks want the Church, and her ministers, to spell it out as a simple matter of, "a Catholic cannot vote for ___" and some even go so far as to say, "a Catholic must vote for ____."
Sorry, that misses the mark, although in ways that may seem subtle.
Let’s start with what’s clear. One may not* formally cooperate with evil. So: one may never support a candidate with the intention of supporting the evil the candidate support.
However, the Church distinguishes between "formal" and "material" cooperation with evil; as well as distinguishing between levels of cooperation, from immediate to proximate to remote. If I hand you the gun and you immediately murdered someone, that’s pretty much "immediate"—and even if I didn’t "formally" cooperate—I didn’t actually share your desire to murder that person—my cooperation is sinful.
But lots of cooperation is more remote, and often cannot easily be dealt with. I drive you to work downtown, knowing you are going out for a date later, and I pretty much know what sins you plan to commit on that date. Am I culpable because I gave you a ride that morning? That would be an example of remote, material cooperation.
Well, these are the principles that apply directly with our political decisions. We are bound to consider the extent to which our political activism is cooperation with evil.
Thus, I think the best formulation has been from Pope Benedict—speaking before he became pope—about how certain issues disqualify a candidate from our support. Some issues are more grave evils than others, and we simply cannot cooperate with it.
But, the problem arises when all the candidates presented to you are disqualified. Then what do you do?
Some would argue, you cannot vote for any of them.
By that measure, then, I would argue we cannot vote for either Obama (because of many issues) or McCain (because of his support for embryonic stem-cell "research"). And, if the Libertarian Party candidate ends up being bad on so-called "gay marriage," then that would disqualify him as well.
But the Church does address this—if the choice of candidates is such that all are disqualified on some non-negotiable issue, then you may choose to vote for one of candidates, as a way to lessen the evil. Note: this means one is not formally cooperating with any of the evils intended by a candidate, but is materially cooperating, either proximately or remotely.
* updated 4:56 pm--sorry!