John McCain isn't done campaigning yet -- he's reportedly gearing up to run for re-election in 2010. His Senate campaigns have typically been relatively easy; like most incumbents, he hasn't faced really serious competition. But this time might be different.
Arizona Gov. Janet Napolitano, a Democrat, will be finishing her second term in 2010, and term limits prevent her from running again. So speculation has long focused on her as a potential challenger to McCain, and the political situation in the state could make the race attractive enough to lure a prominent Democrat. (One of the reasons incumbents often have little trouble getting re-elected is that politicians who think they have a legitimate future ahead of them don't want to risk their careers in races they know they're likely to lose.)
For a little while during the presidential campaign, McCain was in enough trouble that the Obama camp started treating Arizona as a potential battleground. That wasn't really about winning the state, but still, the fact that the Obama campaign could even make a realistic argument that it had a chance in McCain's home state is not a good sign for him.
One poll that came out at the time showed Napolitano leading the incumbent in a hypothetical matchup, 53-45. Granted, that survey was commissioned by the Daily Kos, and it appears to have shared the problem common to the Kos polls -- they tended to oversample Hispanic voters -- but it's still a sign that McCain may not be able to rest on his laurels.