The difficulty of answering that question is that it’s hard to think of any scenario under which John McCain can make up much ground in this debate. The fundamentals are playing into Barack Obama’s favor now, so all he has to do to win is ride the wave. Most Americans believe that Republicans are to blame for the current economic crisis, so as long as Obama keeps reminding the audience that McCain is another Republican, and he’s their only opportunity for genuine change, and avoids a monumental gaffe, he’ll be fine.
As for McCain, he has a much higher hurdle. He has to convince skeptical Americans that he has a plan to deal with the economic crisis that’s distinct from the policies of the Bush era, while Obama’s plans would cause economic catastrophe. The one advantage he has going in is that it’s a town hall format, so he’ll feel at home, and has more opportunities for informal give and take and spontaneous humor that he’s known for.
UPDATE: Apparently, the strict rules of the debate will limit interaction with the audience, which I think inhibits McCain’s ability to connect.