A number of people have asked that question, in light of the most recent batch of polling, in which Gallup’s tracking finds Mitt Romney leading Obama, while multiple other polls find Obama with varying leads.
Ron Brownstein offers a pretty persuasive explanation for the divergence:
Four recent national polls, including three released in the past 24 hours, generally show the electorate dividing between President Obama and Mitt Romney along lines of class, gender and race familiar from the 2008 race.
The surveys-from ABC and the Washington Post; the Pew Research Center; CNN/ORC; and the first Gallup tracking poll, diverge in their overall results. The first three polls show Obama leading by seven, four and nine percentage points respectively; the first Gallup track placed Romney up by two percentage points.
But the Gallup track, which is conducted among registered voters, has a sample that looks much more like the electorate in 2010 than the voting population that is likely to turn out in 2012: only 22 percent of the Gallup survey was non-white, according to figures the organization provided to Emory University political scientist Alan Abramowitz. That was close to the non-white share of the vote in 2010 (23 percent), but in 2008, minorities comprised 26 percent of all voters, according to exit polls; the Obama campaign, and other analysts, project the minority share of the vote will increase to 28 percent in 2012. In its survey, Pew, for instance, puts the non-white share at 25 percent
Brownstein’s conclusion, from all the polling:
Even with their modest variations, these four surveys paint a similar picture. Obama is largely holding the minority and college-educated white women who comprise two pillars of the modern Democratic base (along with young people.) But he is facing erosion among blue-collar white men and struggling to maintain even his modest 2008 support among the two swing quadrants in the white electorate: the college-plus white men and non-college white women. For the moment, that division of allegiances is enough to provide Obama an overall advantage.
The other thing to keep in mind is that demographic shifts within some of the key battleground states themselves could boost the vote share of minority voters, and reduce the vote share of blue collar whites, in those battlegrounds. The details on that can be found in a recent study by the Center for American Progress.
Of course, all this is way premature, and pollsters will be refining their polling over time to account for all sorts of fluctuations and irregularities. And it seems fair to note that Pollster.com’s aggregrate of horse race polls, like Gallup’s track, also finds a dead heat. Still, the overall demographic composition in these surveys is something that will be worth keeping an eye on as we seek to figure out what they are really trying to tell us.