As of Sunday, roughly 15 percent of the national vote had already been cast according to Gallup. Among all Americans who have voted, Mitt Romney actually holds a modest lead over Barack Obama. From Gallup:
Yet, in the important swing states Obama holds a big lead in the early vote. In Nevada and Iowa significantly more Democrats than Republican have voted already. Similarly, polling in Ohio shows Obama leads 63% to 36% among people who have voted.
I often think of the national polling like a “control group” in an election experiment. It is only in the swing states that the campaigns are really active. By comparing what happens in the swing states to what is happening in the non-swing states, you can try to separate broad trends from what is actually being accomplished by the physical campaigns.
The Obama campaign has been remarkably effective at getting its supporters to vote early, but only in the states in which they are heavily invested.
Given that Romney leads nationally with early voters, one can assume that there is nothing about the demographic make up of Democrats that makes them more likely to vote early than Republicans. So the fact that Obama holds a huge lead among early voters only in the swing states strongly indicates his campaign has a vastly superior ground game.