Republican "outsiders" will have an edge. That is going to hurt John McCain (who has been in the Senate since 1986, and was a member of the House for a while longer), and it gives Rudy Giuliani and Mitt Romney an edge. This also makes Governor Mike Huckabee a somewhat viable option as well.
McCain has too many fights with the base to really make it to the nomination... campaign finance reform, detainees, tax cuts, and immigration (that one he was right on) are all areas where he's butted heads with conservatives on a very public basis. Unless Romney, Giuliani, and other candidates split the "not-McCain" vote enough, McCain's an also-ran.