Cranmer argued a couple of days ago that Christians had a duty to vote Conservative. Tom Harris has taken issue with him, and Cranmer responds in the comments thread.
Cranmers main argument was that the Conservatives are more likely to permit a vote on reducing the time limit for abortion. Personally I'm not sure if this is a dog whistle: drop some hints to the pro-life lobby that you're on their side, and they'll queue up to vote for you.
I'm not convinced it's as simple as that. From abortion to global warming to attitudes to money and greed, there are a whole swathe of plumb-lines to hold against our political parties. None of them comes out smelling of roses (or trees, or seagulls, depending on which logo you prefer). Any approach to voting that uses 1 issue as a trump-card to avoid thinking about the rest is sub-Christian.
Update: Cranmer has responded to the Tom Harris piece.