This is about the only scenario I can't imagine. While Sandy slowed Romney's momentum a bit, it didn't reverse it. And clearly Romney had the monentum down the stretch. Yes, it's possible that momentum won't be enough to secure a victory, but it almost certainly would be no worse than a narrow loss.
It's also possible the momentum will be enough for a narrow Romney win, which is what I expect. A fair number of people will see it as enough for a Romney landslide. I don't, but here's hoping. But it's virtually impossible for me to see a big Obama win. Of course, a narrow popular vote win, or even a loss, could still result in a fairly wide electoral win for either candidate, if the chips fall just right(or wrong, depending on your point of view).