There is no organized group of Republicans who will have the yay or nay on running McCain. By appealing to the base, setting up support structures in certain states, and getting an IOU from a lot of House Republicans who he
may help keep in office, he is essentially structuring the party the way he wants it to look.
He's also a media whore, and gets a lot of free press from shows like the Daily Show, who guys like Brownback and Allen couldn't pay to get on.
I think McCain could do well in the South, depending on time and who else decides to run. He's paying lip service to all the right issues, said he'd sign the South Dakota bill, etc.
This is a pretty good article on some of the behind-the-scenes stuff he's doing.