Josh Harris quotes John Frame about single-issue voting.
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.
I haven't read that Frame book, but I really like how he views the question.