One of the more profound insights I've found in the writing of Pope John Paul II, though of course the idea does not originate with him, is that the things that we choose to do always end up changing who we are. This is a profound truth about the human person. Sin brings us closer to Hell because it makes us more the kind of person who will ultimately be at home in Hell. Good works, done out of our own free will with the help of grace, bring us closer to the Beatific Vision because they make us more the kind of person who is close to God. What we choose to do changes us.
A lot of argumentation in the blogosphere, though - particularly political argumentation - tacitly assumes that this is not the case. The notion seems to be that if I vote for a medical cannibal like John McCain or Barack Obama, having decided to do so as a choice of the lesser of two evils, that making that choice does not mean that I will do anything else differently: I will be the same person and do all the same things subsequently whether I vote for a cannibal or not.
But this is obviously not the case. It is not the case for an individual, whose effect on the election is literally negligible. And it is not the case when we aggregate individuals. Five million people who are unwilling to vote for a cannibal are a different kind of group from five million who are willing to vote for a cannibal. Refusing to pull the lever for the least bad viable option is in the end far more powerful on an individual basis than pulling the lever for the least bad viable option, because pulling the lever or refusing to pull the lever changes what kind of person you are. And what is true on an individual basis is true in the aggregate.
"If everyone did it the pro-life cause would lose" is simply false, because it rests on the unspoken assumption that all else remains the same. But all else never remains the same; and most especially we don't remain the same.