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: