Personally McCain’s choice for VP will have a big impact on my decision. There is a high probability that one way or another, he could end up being POTUS. If McCain wins, given his age he could die in office. If not, his VP would be the presumed GOP nominee in 2012 of 2016. If they lost, McCain would take the blame, and his running mate would be the frontrunner in 2012. He needs to pick a conservative who is young enough to be viable four or eight years from now.
I don't want to vote for a guy that I'm going to spend the next four years hoping for him to die - I think that's beyond "near occasion of sin."