(This started out as a response to the a comment left on this post, but it grew long enough to become a post in itself.)
The point of the post was not to argue that people have a moral imperative to vote for a third party candidate. It was more to argue, contra what some bloggers and commentators seem to be arguing, that one has a duty to vote for John McCain.
I am of the opinion that it is very hard to commit a mortal sin by voting, at least for the President, since the electoral system is set up in such a way as to make every vote only remotely connected to the outcome. Unless one formally cooperates with evil by voting for a candidate with evil policies because you agree with these policies, I am doubtful that one can commit mortal sin by voting for a particular candidate.
But I think that one can still commit venial sin by voting. A venial sin is an act that "weakens charity" because "it manifests a disordered affection for created goods" (CCC 1863). I think that too often both Democrats and Republicans convince themselves, out of a disordered affection for their political party, that their candidate is so much better on one set of issues that their incorrect views on other issues are not relevant. I think the "lesser of two evils" approach is more likely to slowly make people comfortable with stomaching evil than it is to help people actually accomplish something good.
For example, why do people stomach the Republicans simply because they say they will appoint pro-life judges? The current Supreme Court is consists almost entirely of Republican appointed justices. Should Roe v Wade not have been overturned by now? And why was Ron Paul's H.R. 300: We the People Act killed by being sent to committee? The Congress of the United States has the Constitutional authority to limit the Federal courts' jurisdiction. Why have the Republicans not fought to do so for important life issues? Is partisanship for the GOP really the answer? Or does it do nothing more than habituate people to voting for the GOP no matter what their candidate believes? Why do some pro-life groups recommend voting for GOP candidates over Democratic candidates even when both are pro-abortion? Perhaps, just perhaps, pro-lifers have been sold a bill of goods, or at least told that our issue is more important than it seems to be when our elected officials are done canvassing for votes and are ready to get down to the business of actually governing.
So, at least for me, the question does not come down to "what is not mortally sinful?" but rather to "what is virtuous?" I do not think I can virtuously vote for John McCain. It is true, of course, that politics is often "the art of the possible." But it is also true that prudence is the guiding virtue of the active life, and thus it is the guiding virtue of the political life. I am currently of the opinion that I cannot prudently vote for John McCain for the same reason that I could not have voted for many of the other GOP candidates. They were not all that bad. Not great mind you, but on many of the issues I care about they were not all that bad (the exceptions being Rudy Giuliani, who was horrible, and Ron Paul, who was great). But this is exactly the problem. I have been willing to make the concession of voting for a "not great" candidate before, and all it did was make me more attached to the party and the candidate. I would react viscerally against any criticism leveled against them, even if said criticism was something I knew to be true and was delivered without malice by people I had reason to trust. This is not virtuous. I still have problems with this, even though I am, at least intellectually, thoroughly disenchanted with the GOP. This is not virtuous. It is not acting in accord with right reason. It is acting in accord with irrational emotions born from a disordered affection towards a political party. If I were to continue voting for Republican presidential candidates because they were "good enough" or "better than the alternative" I would simply keep habituating myself to the same kind of disordered affection.
For this reason I made the choice a few years ago to only vote for candidates whom I believe to be the best choice, regardless of how likely they are to win. Only by doing this can I do my part to work for justice and the common good without being in danger of damaging the supernatural virtue of charity by developing disordered affection for a political party that does not do everything it can to make a difference on the issues that are of supreme importance. If nothing else, my vote can register as a protest against the way the two-party system currently operates. It may only be a drop in the bucket so far as notice goes, but it was only a drop in the bucket anyway.