Principle and Pragmatism in Politics

I recently listened to a broadcast of Al Mohler on his radio show. I think it was the last "Ask Anything Wednesday" episode in which he answers questions from callers.

One caller asked him about the possibility of him supporting a third party candidate (Constitution Party). Mohler responded by talking about principle and pragmatism. He first said that each believer should make up their minds the first tier issues... On these issues, Mohler said that Christians should decide beforehand those candidates they would not vote for as a matter of principle.

But then, once the candidates for the two main parties have been chosen, he seemed to be saying that Christians should weigh their vote pragmatically. He concluded by saying that there's very little possibility of a third party candidate winning. Mohler didn't give a "therefore" after that last statement, but it seemed clear to me that his conclusion was "Therefore, I wouldn't vote for a third party candidate."

Is it just me or does it look like pragmatism wins out in this situation? I've wrestled with this issue before and am open to reasonable answers, but I just don't see it for now. If the election was tomorrow and I had a choice between Giulianni? or McCain vs. Hillary or Obama, I would vote for someone else. Is that unreasonable?