I'm not sure I really agree with the consensus here: McCain's approvals are TERRIBLE in non-joke polls. And unlike Mitch McConnell's shocking double digit wallop, the Democratic candidate is very solid and 2016 is not prone to be a 2014 scenario. I think it's equally likely Dems pick this seat up whether or not McCain is the nominee.
McCain has already announced that he will campaign mostly on local issues. That usually works. And unlike other Republicans, he will do better among Hispanics. Ask Jeff Flake why he almost lost in 2012... because Carmona crushed him among Hispanics. I despise McCain, but there is no guarantee that an alternative candidate will be a better campaigner than him. In fact, it is really unlikely. McCane has the incumbency advantage on his side - just like Ayotte and Toomey.
Incumbency can turn into a disadavantage - There's a reason why Chris Dodd, George Voinovich, and Joe Lieberman didn't run for re-election.