From John Fund on WSJ.com:

He may have been his party’s presidential nominee last year, but John McCain could still have to endure a primary challenge from the right when he runs for a fifth term in the U.S. Senate next year. Conservative radio talk show host J.D. Hayworth, who was a Republican member of Congress for a dozen years until losing his seat in 2006, is mulling running against Mr. McCain.
Mr. McCain has always had a tenuous relationship with the grassroots of his party in Arizona. Many GOP voters remain genuinely angry with him for cosponsoring a comprehensive immigration bill with the late Senator Ted Kennedy in 2005. A new Rasmussen Reports poll finds that 61% of Arizona Republicans think Mr. McCain has lost touch with the base of his party. In a general election, however, Mr. McCain remains a shoo-in since his approval rating among moderates is a striking 64% and he also wins approval of a surprising 41% of liberals.