With Obama's considerable ground game advantage over McCain, and the latest polls showing a 10 percentage point lead, the Illinois senator is expected to benefit the most from early voting.
In some states, such as California, experts estimate that up to 40 percent of voters are going to vote by mail. In Oregon, everyone votes by mail.
Battleground states like Nevada, New Mexico and Colorado -- key states Obama is hoping to win -- have typically had high rates of early voting.
"More than half of those ballots will come in early," Gronke said.
States like Florida and North Carolina generally have between 20 percent to 30 percent early voters. Wisconsin and Minnesota have typically had lower levels of early voting, as has Ohio.
"Ohio is going to be different this year because they changed their rules and regulations and made it much, much easier for people to cast an early ballot," Gronke said.
In 2004, 6 percent or 7 percent of the ballots in Ohio came from early voters, but because of the new "no-excuse" provision, that number is expected to be much higher this year, Gronke said.
The Ohio Republican Party sued unsuccessfully in federal court to stop a provision that allowed early voters to register and cast a ballot on the same day in Ohio. From Sept. 30 to Oct. 5 -- dubbed "Golden Week" by some progressive advocacy groups -- Ohioans were permitted to register and vote on the same day.
The GOP had argued that because the early voting window allowed voters to register and vote on the same day, officials wouldn't have enough time to verify registration information.
Hightower predicted early voting will increase the number of Ohio students who vote this year.
"There's a lot of passion on campuses and intention to vote but any number of things can come up whether it's a test, work or class that might derail the students to go," he said.
"More people want to vote this year and care about the election," Hightower said. "So whenever they have the opportunity to do it on their own schedule, to do it when it's convenient to them, it just provides a greater window for them to take advantage of voting."
Gronke said more and more people are voting early because they can.
"More states have made this available, and citizens are responding," Gronke said. "States believe that this will help them deal with the crush of voters that appear on Election Day."
Many states adopt early voting after a "crisis," Gronke said.
"Florida adopted early voting after 2000 and Georgia followed suit," he said. "Ohio relaxed their 'no-excuse' provisions after 2004, so sometimes these laws are adopted rather quickly because it's seen as a way to respond to a problem in the election system."