It's beginning to dawn on me that all this talk about Romney the Centrist--about how he'll move to the center now that the primaries are behind him, that he'll govern from the center: all that is just a mirage. Romney may not be a wingnut himself, but he is in, of and for the party of the wingnuts and that is the way he will play it out as far as the eye can see. There are all kinds of reasons why we can expect this to be true.
First, as others have noted, this is not a campaign of enthusiasms. Neither side likes its candidate all that much. They won't vote for him so much as they will vote against the other guy. If that is your strategy, you've got to go negative and stay there which in the nature of things is likely to to drive you to the extreme.
Second, all that stuff about governing from the center in Massachusetts--well, maybe he did but that is becaaue Massachusetts is a sorta kinda centrist place. There's nobody who remotely fits that model in Congress. One thing about Romney, he does like to make deals, so he'll go where the deals are. Which explains why he is finding it easy to make kissy face with Paul Ryan, and while he'll do the same with Mitch McConnell and Eric Cantor.
Third, infrastructure. A candidate isn't an individual; he is the nucleus of an organism, a survivor in an ecological niche. Practically speaking, Romney can't begin to run a campaign--or an administration--without Liberty University and the Heritage Fund because that is all the Republicans have left.
Those are three reasons that come to me off the top of my head. I assume there are more?