IMHO, I keep going back to McCain as the Perot in 2004. He grits his teeth and smiles as he professes to support GWB, but his hatred for the man is so obvious. He undercuts the Administration every chance he gets. Note the latest comments the other day. I have a feeling that he will leave the GOP and run as an independent so as to prevent a second term for GWB at any cost, no matter how detrimental to the country.
By now, I think McCain would win more votes from the Rat side of the aisle than the Pubbies. Most Republican GWB supporters are wise to him and simply tolerate him. Arizona sure knows his tricks.
He would lose every primary he ran in (with perhaps the exception of those open primaries, and even then I think he would lose, because the democrats need to vote in their own primaries this time).
His major danger would be as an attack dog on Bush and the sound bites he would get on the evening news.
I don't think he has enough support to mount a campaign, however. It costs money and I don't think he has any.