First, Gronke stresses that it's a misconception that most early voters will miss out on all of the debates: In Florida, for instance, early voting doesn't start until Oct. 27, just a week before the election. So while early voting is extremely popular in certain states—in Nevada and Colorado, it accounts for more than two-thirds of the ballots cast—many of these voters will actually be waiting until late October to cast their ballots. Right now, "very few people are actually casting ballots," Gronke says.
What's more, there are certain voters who are more likely to vote early than others: those who are older and more highly educated, and who tend to be more set in their political beliefs, he adds. So early voters are more likely to be Republican, given the demographic make-up of the GOP's base, regardless of who will ultimately win the election. (Early voters in key swing states, however, tend to vote Democratic.)
That said, Gronke believes that early voting patterns can reveal some important trends that both presidential campaigns will be paying very close attention to: Early voting is an important indicator of enthusiasm, and the campaigns will be watching for any divergence from previous years' early voting patterns. In 2008, African-Americans in the southeast turned out in huge numbers to vote early for Obama, and a sharp drop in early voting by those voters could be a warning signal to the campaign.
What's more, campaigns can comb through hugely complex voter databases to see whether individual voters who've tended to vote early in previous election aren't showing up this time around. So by "comparing voters' past behavior to their behavior right now," campaigns can decide, for instance, to dispatch staff to knock on certain doors, Gronke says.
Overall, Gronke expects that early voter turnout to be higher than in previous years, particularly as polling shows that the number of undecided voters is quite small. That said, "people may be a little bit less enthusiastic about their choices"—Romney may not be their ideal GOP candidate, and Obama may have lost some of his 2008 mojo—so that could dampen some of the appetite for rushing to the polls before Election Day.
My colleague Aaron Blake has created a great early-voting tracker that will keep tabs on the early ballots, updated regularly with new data and analysis.