Proportionate reason and utilitarianism

Here's a question, the very one at the root of this new found enthusiasm for voting for candidates who favor unrestricted abortions, it has to do with proportionate reason: What is the difference between proportionate reason and applying a utilitarian calculus?

In posing this question I suppose I am curious about axioms like, politics is the art of the possible and diplomacy is the art of compromise. I am not belittling either of these axioms, far from it, there is some wisdom there and this wisdom finds its Christian roots both in our Lord's admonition to render to Caesar what is Caesar's (Mk 12,17 ; Matt. 22,21; Lk 20,25) and St. Augustine's two kingdom theory, fully expounded in his great work City of God, and reiterated in Fr. Neuhaus' presentation last Saturday.

I suppose in this question, or the general exploration of proportionate reason, I see the crux of all the issues Fred mentions, as well as what our bishops call on us to use when deciding for whom to vote.