The key is in enthusiastic gap. In 2004, Obama won Ohio by only 4.6%. there was one rally with 80,000 people attending and a lot of college kids were volunteering for him. Yesterday, Romney had a rally with 30,000 people in Ohio but Obama had only 2800. You can see the big difference this time.
Since Gallup suspended polling due to superstorm sandy, most remaining polls are very partisan (actually some of them can be considered as a part of the campaign of either side) and you cannot trust them. Let's assume O leads R by 2% on average. R still can win if they can motivate 5% more voters to the poll.
Poll% | Vote% | Weigted% | Final% | |
Obama | 50 | 0.85 | 42.5 | 49.6% |
Romney | 48 | 0.9 | 43.2 | 50.4% |