At this point, McCain, who lost to Bush in a bitter 2000 Republican primary, is in the early but unmistakable stages of laying the groundwork for another campaign.
In addition to McCain, likely contenders include Senate Majority Leader Bill Frist of Tennessee, Virginia Sen. George Allen, Massachusetts Gov. Mitt Romney, former New York Mayor Rudolph Giuliani and former House Speaker Newt Gingrich.
http://www.newsmax.com/archives/ic/2005/6/7/91918.shtml
The real question - if McCain loses the republican primary, will he then run as an independent? If he does, then whoever the Democrats run will certainly win, and they will run Hillary.