Nate Cohn looks at Gallup's demographic assumptions:
A few days ago, Gallup released a demographic breakdown of its likely voter poll and simple algebra demonstrates that white voters represented 80 percent of Gallup’s likely voter universe, up from 77.5 percent in 2008. It's not hard to imagine how the electorate could be less diverse than it was four years ago. Obama’s candidacy generated historic and potentially difficult to repeat turnout from African Americans and the young voters who allow Democrats to capitalize on demographic changes. Outside of the battleground states, the decline in enthusiasm could be especially large.
But there is reason for skepticism:
Gallup’s likely voter universe is actually even whiter than their likely voter surveys prior to the 2010 midterm elections, which was 79 percent white. This observation is likely to produce one of two responses, with Democrats all but assured to assert that the poll is demonstrably wrong and Republicans taking it as confirmation that Democratic enthusiasm is down, particularly among the non-white voters who brought Obama to victory four years ago.
Nate Silver examines Gallup's record:
In 2008, the Gallup poll put Mr. Obama 11 points ahead of John McCain on the eve of that November’s election. That was tied for Mr. Obama’s largest projected margin of victory among any of the 15 or so national polls that were released just in advance of the election. The average of polls put Mr. Obama up by about seven points. The average did a good job; Mr. Obama won the popular vote by seven points. The Gallup poll had a four-point miss, however.
He notes that Gallup also missed the 2010 results by roughly 8 points. Blumenthal weighs in:
Gallup's results are very different, at least for now, compared to most of the national polls, just as they were in 2010. The best advice may be what political scientist and blogger Jonathan Bernstein offered his readers: As "with every polling number," he wrote," "ignore it, and look at the polling averages."
(Chart from Gallup)