Obviously, the 2012 election is more than 13 months away, and the contours and strategies of an Obama general-election fight with former Massachusetts Gov. Mitt Romney would look very different from an Obama matchup with Texas Gov. Rick Perry or, for that matter, Rep. Michele Bachmann of Minnesota. Against Perry, Bachmann, or any number of other very conservative candidates, the Obama campaign would clearly focus on moderates and independents, the classic swing voters. But against a Romney, or if lightning were to strike and former Utah Gov. Jon Huntsman somehow captured the GOP nomination, the middle might be a much less hospitable place for the Obama campaign to search for support.

Simply put, a less ideologically driven challenger allows the race to be more of a referendum on the Obama presidency, while a full-throated conservative challenger makes the contest more of a fight between two dramatically different visions of government and where to take the country. With the economic situation just short of total disaster, we have a pretty good idea how the referendum question would come out; the competing visions would make for a much more interesting contest.

If Romney ends up as the Republican nominee, Obama's campaign would have to go full tilt to try to get the kind of turbocharged voter turnout that gave him his comfortable victory in 2008--only this time, with the incumbent highly unlikely to win the independent vote by anything like the 8-point victory he achieved last time, his margin for error would be slim or none. In the Gallup Poll aggregate tracking data for the week of September 19-25, Obama's job-approval rating among all independent voters was 36 percent (compared with 78 percent among Democrats and 7 percent among Republicans). Among "pure independents," those who if pressed don't lean toward either Democrats or Republicans, his approval rating was just 31 percent, having ranged from 27 percent to 32 percent in recent weeks. (Independents who lean toward a party generally vote that way; it's the pure independents who really swing.)

One complicating factor is that independents are harder to ignore than they once were. In Gallup polling between Labor Day and Election Day in 2004, the broader group of independents made up between 29 percent and 32 percent of all Americans. During that same period in 2008, independents represented between 31 percent and 38 percent; since May of this year, they have ranged from 37 percent to 44 percent. Republicans have ranged from 24 percent to 30 percent since May, while Democrats have run as low as 26 percent to as high as 34 percent. While some can quibble and can cherry-pick data from past elections to downplay the significance of independent voters, the trend is pretty unmistakable: Voters who identify themselves first as independents are now the biggest bloc; that share is growing, and Democrats and Republicans trail.