We're obviously never going to know exactly what happened to Obama's apparent 8-point lead in New Hampshire, but my colleague John Judis has hit on an explanation I think will hold up even after the pollsters and political scientists have picked over the data. Judis argues that it was a combination of: 1.) women, especially educated women, fleeing John Edwards and (to a far lesser extent) Barack Obama for Hillary Clinton, and 2.) An Iowa bounce for Obama and (to a lesser extent) Edwards that was most pronounced among working-class people--who hadn't previously been paying much attention--and dissipated almost as quickly as it appeared.
Here's the key graf on the latter theory, which has been mostly overlooked in this debate:
The Gallup poll included Friday, the day after the Iowa caucus, when Obama's totals were most likely to be affected by an immediate boost from Iowa. (This happens invariably after a competitive Iowa caucus--for instance, in 1976, 1980, 1984, 1988, 1996, and 2000.) And if you compare it with previous polls before the Iowa caucus, the boost seems to have come even more among lower than among higher income voters. These voters had previously been one of the bulwarks of Clinton's vote in New Hampshire, as measured, for instance, by the December 23 University of New Hampshire survey. The Gallup poll not only showed Obama doing unusually well among these voters, but also showed him overall with 41 percent total, and 13 percent ahead of Clinton, and Edwards with 19 percent and closing in on Clinton. So if you were going to ask why this poll didn't predict outcome, you would look mostly at how early it was taken, and how much it was affected by the Iowa results.