I think Paul at Powerline has a good plan.
Obama is saying that, as far as he’s concerned, it’s full speed ahead with the debate. McCain’s best move is probably to show up and debate. McCain can observe that he preferred to spend the time working to solve the financial crisis, but that Obama felt otherwise, and he (McCain) is not one to back out of a clash. McCain can also point out that the Obama who now thinks a given debate on a given night trumps dealing with a major crisis is the same Obama who wouldn’t accept his challenge to a series of town hall debates during the summer.