I think that it all depends on the spin. IF (and this is a big IF) McCain can get his message out fast enough, and loud enough, and be perceived as taking the "High Road", then he just sealed the election.
However, if Obama is allowed to grab the focus...McCain just fumbled the ball inside his own 20. It won't break him, but he'll lose whatever small edge he has right now.
McCain goes to DC to help with the bail-out. Harry Reid obstructs him at all turns to prevent him from making any progress, thus making him seem inept and unable to affect change.
Meanwhile, BO goes to the "debate", gets softball questions, puts words into McCain's (absent) mouth with no one present to contradict him.
When McCain is through in DC, he approaches the Obama camp for a re-scheduling of the debate, to which OB says that he was at the debate --- and, McCain did not show up.
There will be no debate between the two of them, and BO is the big winner.
How's that?