One of the most mystifying aspects of McCain's messaging is why he hasn't run on the "different kind of Republican" theme. This was Bush's message in 2000, even though the Republican brand wasn't as badly tarnished then as it is now and Bush was actually less different than McCain is.
The McCain campaign probably doesn't want to cause more base problems than they already have (see: enthusiasm gap). But you can substantively be a "different kind of Republican" in a DeMint/Coburn/anti-corruption type of way, while using the terminology as a code word to moderates and independents.
How can McCain be a different type of Republican and help himself with the base? Simple. Throw Ted Stevens under the bus.
As the party leader, McCain should be unafraid to set an example for other Republicans, particularly because of his vocal stands on campaign finance and his previous clashes with Stevens. If McCain calls on Stevens to resign, and Stevens steps down, McCain looks the guy who did it. If Stevens doesn't resign, McCain looks like a maverick, and differentiates himself from a corrupt Republican and politics as usual.
Stevens stepping down is an unalloyed good for the party. Blasting him is good with the media and independents. The only reason not to do it is harming his relationship with other Republican Senators, but it's not like we're going to be in the majority next year anyway. As far as McCain goes, this should be a win-win.