The problem with McCain is no one is really sure how he is going to vote on issues (except maybe Iraq) and that scares both the Left and the Right.
I compare him to Liberman, unfortunately, as they both vote moderate once in a while and then their 'own team' doesn't like them.
Only ones that are trusted by their own Party as those that vote the Party line. The trick is to make yourself look moderate while not alienating your own Party, one thing Hillary for example, has completely failed at recently.