Your question would have more meaning if you weren’t actively acting to help return to office a guy who is even more egregious. I’d hate to have to stand before God and explain that I helped re-elect Barack Obama because I was more concerned about stumping for a preferred political candidate than I was about using the common sense that He had given me.
Next.
Well, the problem with this argument, at least when trying to use it with me, is that I can turn it right back around. If I were only going to vote for someone I knew to be a Bible Christian with correct doctrine, then that would mean Santorum and Gingrich are out (Catholic), Sarah Palin is out (Charismatic), Perry is out (non-denominational come as you are no-standards), Huckabee is out (social gospel "Baptist"), and if I really wanted to investigate thoroughly, I'd probably conclude that even Herman Cain and Michelle Bachmann were unsupportable because they use the wrong Bible version or believe in the universal church instead of taking a local only position. Romney being a Mormon makes him no more problematic than would be Ryan, Gingrich, and Santorum, who are all Catholics, which I believe is also an unscriptural cult.
How do you rectify this behavior with the Bible's message to warn people of their sin and if they continue to not listen, have nothing to do with them?
By the fact that those commandments are not referring to politics, but to spiritual matters, especially as those pertain within a local church body, the body of Christ. In fact, trying to apply these to politics in the way you are doing is gross worldliness, as I see it.