Reason  No 1 : At the end of September, Obama led his opponent in essentially all credible opinion polls.

Romney was caught in  a controversy around the “47 percent” tape.

And then the first debate happened.

With a strangely passive debate performance, making no mention of the “47 percent” video and even suggesting he and Romney agreed on the the future of Social Security, Obama gave the Republican a massive new opening.

Romney's swing-state numbers  surged, and in national Pew polling, he went from an 8-point deficit to a 4-point lead over Obama.

If Romney emerges the victor, some of the Democrats will say that the reason is really very simple: Obama self-immolated on a single night at the University of Denver.