Gallup might be showing a closer race than most other polls of registered voters, but they still show evidence of increasing Democratic turnout. According to Gallup, similar proportions of Democrats and Republicans and whites and African Americans now say they will definitely vote. While Democratic turnout continues to lag slightly behind the Republicans, the gap is no longer substantial enough to threaten Obama's modest lead.

But some Democratic-leaning demographic groups, like Hispanics and young voters, still seem poised to turn out at lower rates than the rest of the population. Today, just 64 percent of 18-29 year olds say they will definitely vote, compared to 78 percent in October and November of 2008. If youth turnout fell by 18 percent, that would cost Obama a net one million votes nationally, or about one percent, provided that young voters support the president by a 63-37 margin. These turnout concerns aren't especially significant if Obama can solidify a 5 point lead, but it could make a difference if the election becomes extremely close. 

One critical and unanswered question is whether stronger turnout in the battleground states might disproportionately benefit Obama. Given that most of the lower turnout groups appear to be relatively favorable to the president, it's possible that higher turnout might explain Obama's slightly larger edge in battleground state polling.