Remember the first debate in 2004? George W Bush was practically incoherent.
First 2004 Presidential Debate
Kerry, of course, went on to lose that election, despite Bush appearing almost unqualified to be President. Watch that tape and compare to Obama's performance, which was not even close to being that bad.
This will probably move the needle 2 percent in Romney's favor. The Vice-Presidential debate will move the needle none (ask Vice President Bentsen about that).
The next debate is a Town Hall format, which favors Obama - I'll be shocked if Mitt gets through it without insulting someone (and you know he'll get a 47% question from the crowd). The foreign policy debate will favor the guy who killed Osama Bin Laden. Romney is out of his depth.