Nate Silver's latest update looks at the "essential role" of Ohio, and this election:
Were he to lose Ohio, Mr. Romney would have a number of undesirable, although not impossible, options. The most favorable path, in the view of the model, would be for Mr. Romney to carry both Iowa and Nevada.
Of the two states, Iowa is the easier get. The polls there show a split between ties and leads for Mr. Obama, as opposed to Nevada, where they are mainly split between smaller leads for Mr. Obama and larger ones. In addition, in Nevada, Democrats have a significant voter-registration advantage and are building a large lead in early voting; the polls there have also tended to underestimate Democratic performance in recent years.
But Mr. Romney does not get to pick and choose if he loses Ohio; he would need to win both Iowa and Nevada under this plan. Furthermore, he would need to win New Hampshire to avert a 269-269 tie, where the polls have been inconsistent at best, but seem to show Mr. Obama slightly ahead, on average. On top of all that, he would need to win both Colorado and Virginia.
There are a few less likelier options, but as always from Silver, a fascinating read.