A number of bloggers are castigating McCain for even thinking of rubbing shoulders with perhaps the most despicable and slimy politician on the planet. A chickenhawk who smeared as unpatriotic a war veteran that had lost three limbs in Vietnam, and McCain himself said in 2002 that the ads were reprehensible. So why is he helping this guy out? He's not needing to cozy up to the dark side of the party anymore, which might explain him lending a hand here.
To me it boils down to a seat at the big table. If Chamblis is re-elected that will keep the Dems from having the 60th seat. The two other contested races appear at present to being going to the Dems, but that is far from certain. If the senate is split 59/41 it's not filibuster proof, and those pesky neocons can stonewall for two years, the two most important years of Obama's presidency, particularly if he's wanting to be re-elected. That mean Obama needs to woo one or two votes his way in order to get legislation passed, judges appointed, cabinet members approved. He needs McCain to cross the aisle, and he needs to make nice to Leiberman, no matter how distasteful that may be. Now if that walking pustule loses the run off election then Obama doesn't have to let McCain sit at the big table on the important issues. This would relegate McCain to obscurity. It's bad enough he lost his bid for the White House after selling his soul, but to be cast aside for the rest of his political life and finish it out in obscurity would be horrible.
Anyway, that's my take.