Dr. John Frame is a professor at Reformed Theological Seminary in Orlando where I am currently enrolled as a D.Min. student.  He has this to say about Christians and political choices that I find very helpful:

“…in some cultures (like the ancient Roman, in which the New Testament was written) there is not much that Christians can do, other than pray, to influence political structures and policies. But when they can influence them, they should. In modern democracies, all citizens are ‘lesser magistrates’ by virtue of the ballot box. Christians have an obligation to vote according to God’s standards. And, as they are gifted and called, they should influence others to vote in the same way.

This is not to say that political choices are always obvious. Often we must choose the lesser of two evils. Candidate Mershon may have a better view of one issue than Candidate Beates, while Beates has a better view on a different issue. It is an art to weigh the importance of different issues and to come to a godly conclusion. Each of us should have a large amount of tolerance for other Christians who come to conclusions that are different from ours. Rarely will one issue trump all others, though I must say that I will never vote for a candidate who advocates or facilitates the killing of unborn children.”-John Frame, The Doctrine of the Christian Life (P&R 2008). p. 617.

(HT:  Josh Harris)