Nate Silver notes the significance of Gravis Marketing's rather ill-timed (because conventions are destabilizing) Ohio poll, which shows Obama leading Romney "by less than 1 full percentage point":
[W]hy does this qualify as good news for [Obama]? Because this firm has had Republican-leaning results in the other states that it has polled.... Once the model adjusts for the firm’s "house effect," it treats Mr. Obama’s nominal 1-point lead as being the equivalent of a 4- or 5-point lead.
What's more, Ohio, when it comes to "electoral calculus," means more than one state:
Ohio has a 30 percent chance of being the tipping-point state, meaning that it would cast the decisive votes in the Electoral College. That’s as much as the next two states on the list, Florida and Virginia, combined. It’s also as much as Colorado, Nevada, Iowa, Pennsylvania, New Hampshire, Michigan and North Carolina combined.
I have yet to read an electoral-count analysis of a Romney victory that's anything than other an almost fantastic collection of ifs and maybes, whereas an Obama victory is virtually assured by even the smallest slip by Romney. And as the saying goes, Nobody's perfect.