The most popular narrative will be voter suppression, although it will be difficult to find someone who was actually prevented from voting. Instead, it will be alleged that many people did not vote because they "believed" they would have been prevented from voting if they showed up at their polling place.
But if he does lose, which is possible, it'll be the first debate that did him in. Obama was looking at cruising into a 320+ electoral victory and easily carrying Virginia, Ohio, and Florida. The Obama campaign was thinking about expanding the map into Arizona.
After the first debate, Virginia was lost to him as was Florida. Ohio and Wisconsin went from a given to a real fight. Obama went from having multiple paths to 270 that were likely, to one truly likely path.
That's why I don't see Democrats going especially nuts after this. Obama clearly brought his current electoral woes on himself. I still suspect he'll win, but it'll be a very close finish. Should he lose, the Senate is still likely to stay "D" thanks to Akin and Mourdock, so nothing will get done on the GOP's agenda. "D" voters will just rally around Clinton in 4 years.