Peter Leithart is one of the sharpest writing theologians around today (imho).  At  recent post on his blog he asks a simple question: what is voting for?  His answer is intriguing.  He writes: "By our votes, we endorse a particular vision of national good that we want to see realized.  Our votes declare what role we think civil power should play in our national good."  He does not quite get to a difficult question: what if no candidate appears to offer a compelling vision of the national good that we think ought to be realized?  You can read his whole piece here.