I've seen a few authors suggest that only 5% of the population is persuadable at the moment, which is why poll movements are so minor from poll to poll. Assuming 2.5% of that belongs to each candidate, Romney would need to get about a fifth of uncertain Obama supporters to bolt.
Approaching this from a different angle, look at the map here:
http://www.cnn.com/ELECTION/2012/eca...#?battleground
Obama is up 4% in New Hampshire and Nevada, and he's not going to lose Wisconsin. From the remaining states, if Obama wins either Ohio, Florida, or Virginia, he gets reelected. If he wins Iowa and Colorado, he gets reelected even if he loses Ohio, Florida, and Virginia. Romney would have to win virtually every contested state in order to get 270 electoral votes.