That analysis is fine as far as it goes, but it still presents the campaign with a problem -- as was made clear by a poll from Suffolk University and USA Today released on Wednesday. Numbers-crunchers have been predicting for weeks that Romney would gain ground once pollsters switched from polling all voters to just likely ones, which they do closer to the election. The Suffolk poll seemed to confirm that. Among unregistered voters, Obama led Romney nearly 3-to-1, 43 percent to 14 (23 percent said they'd opt for a third-party candidate). Among unlikely voters, his support was twice as strong as Romney's, 43 to 20 percent.

And these aren't just the average apathetic people who don't care about politics and think the government doesn't affect them. Most said that politics does have an impact on their lives, and that they pay attention to it. But they still said they don't intend to vote. Twelve percent said they didn't think their vote mattered; another quarter said they were too busy. (That's one reason Democrats are in a tizzy over curtailed early voting laws. For example, Ohio's attorney general said polling places won't be open on weekends this year, which Obama and his allies say disadvantages working people who can't take time from their jobs to cast votes).

It's just those sorts of people -- sympathetic to Obama, unlikely to vote -- who helped him win four years ago. Voter registration deadlines vary across the country, but they're approaching throughout September and October. Even if unlikely voters register, they and those who are reluctant to vote presumably pose a serious challenge for the campaign later on. Of course, that's assuming the campaign has actually been trying to register voters. Sasha Issenberg, the best reporter on the data-driven campaign, reports that Obama's team is essentially treating voter-registration drives as a way to build volunteer networks for later in the campaign, not so much to, you know, increase registration.

There aren't many undecideds left in America. What this new crop of data shows is that the challenge for Obama isn't to win them over -- it's to get the unlikely voters to vote. That's not a mission that inspires a lot of confidence, but perhaps that fabled ground game is just keeping its powder dry.