The electoral map is changing—and changing fast. Just a few weeks ago, Obama was nearing 270 electoral votes. The president appeared to have 265 electoral votes as opposed to 191 for Mitt Romney. As of this morning, however, Obama is only leading by 10 electoral votes, according to Real Clear Politics—201 to 191.

With 146 votes now in the “toss up” category, there are obviously a number of different routes to victory for each candidate. But the most likely path for both men seems increasingly to revolve around Ohio.

It now seems more and more likely that Obama will lose Florida, where Romney has taken a 2.8-point lead. Romney also leads in North Carolina (by 4.7 points) and in Colorado (by 0.6 points). Though Obama leads in Virginia by 0.8 points, the trend line is against him there—as recently as mid-September, he was winning by nearly 4 points.

Obama can afford to lose these states if he holds Pennsylvania (where he is up 4.5 points), Michigan (where he leads by 4.4 points), Wisconsin (where he is up 2.3 points), Nevada (where he is up 1.6), and Ohio (where he is up 2.2).

Democrats have won Wisconsin every year since 1964, and our guess is that the pattern will hold there this year as well. We would also give Obama good odds in Nevada. As Harry Reid knows well, Nevada has a sizeable percentage of Hispanic voters, who are mobilized and organized. The demographics have changed in Nevada in recent years, and they heavily favor the president.

That leaves Ohio as the place that is probably the key—for both sides. No Republican has ever won the presidency without carrying Ohio. Romney’s team is certainly aware of this. In the past two weeks, his campaign has aired 12,250 ads in the Buckeye State, even more than they ran in Florida. And, during that time, Romney has held seven events in Ohio.

Obama’s campaign is also making a hard play for Ohio. In Cleveland alone, Obama and Democratic groups have aired 2,717 television spots in the last two weeks. Meanwhile, the president is sending his most powerful, and popular, supporter to the state: Bill Clinton will host a rally in Parma, Ohio, alongside Bruce Springsteen, on Thursday, two days after the second debate. Although Obama will not be there himself, sending this duo, especially Clinton, underscores the importance of Ohio to the race.

Despite the erosion in Obama’s lead, he is still winning in the Electoral College. But his electoral-vote margin has narrowed considerably. And unless something unpredictable happens (as it very well could), Ohio is probably going to be the state that makes or breaks his reelection bid.