I think Andrew has this just right:
A lot of the [post-debate] polling shift comes from a new enthusiasm gap created by the president's historically unprecedented forfeiture of a debate. What Joe has to do is to get Obama supporters' heads back in the game, to remind them of how they felt when Bill Clinton ripped a gaping hole in Romney's hooey, if you'll pardon the expression.
It's the "remind them how they felt" part. That's it. This is why I wrote what I wrote this morning about Biden needing to reset and reframe and tell the Democrats' economic story of the last 12 years. Why 12 years? Because it's absolutely crucial to remind people that right-wing policies failed the country horribly, which three-quarters of the country agreed on the day Bush left office, and that the other tickets wants to put those policies into place again.
Telling that story is in essence what Clinton did at the convention. What Obama backers need right now is a shot in the arm. Biden has to provide that.
It's interesting to watch people's varying degrees of panic. For my part I'd say that I've mostly worked through the panic cycle. I don't think Romney is quite good enough to sustain the debate level for a month, and I don't think Obama is quite bad enough to do the same from the other end. Very few, maybe surprisingly few, liberals I talk to now think Obama is going to lose. What people mostly think is that if he'd won the debate, this thing would be basically over, and they're distressed and angry about that. They're picking themselves up off the floor, but they need a pick-me-up. That's Joe's job.