At Commonweal, William J. Gould advocates a plurality of approaches to voting among Catholics:

The kind of pluralism I have in mind would range from radical perspectives such as that of the eminent Catholic philosopher Alasdair MacIntyre—who contends that the two major parties are so defective that not voting is actually preferable to voting—to support for antisystem third-party candidates like Ralph Nader, to voting for Obama (as I will) on the grounds that, on balance, his administration will do more to serve the common good than McCain’s, to voting for McCain (as many others will) on prolife or other grounds.

As a non-Catholic, I obviously don’t consider myself bound by the Magisterium, but I agree with Gould’s general approach: Christian principles don’t yield a single approach to politics. Some might say that means that Christian principles are vacuous if they’re compatible with such a diversity of approaches. But I’d contend that it’s because these principles are rather general and don’t necessarily entail detailed policy prescriptions and because in the real world we’re often required to make trade-offs among competing goods.