With voters in the swing state of Iowa today joining those in two-dozen other states who can already cast their vote for president, the surge in early voting is necessitating a change in campaign strategy, says Paul Gronke, director of the Early Voting Information Center.

Gronke tells NPR's Morning Edition that he expects some 35 percent of all votes in the race between President Obama and Mitt Romney to be cast before Election Day on Nov. 6, even though some states this year have limited early voting.

Gronke, a political science professor at Reed College in Portland, Ore., estimates that up to 33 percent of voters cast early ballots in 2008, compared with about 20 percent in 2004 and 15 percent in 2000.

"I think campaigns have to mobilize over a longer period of time," Gronke says of the changes caused by early voting. "We don't really know whether those last-minute bombshells that ... don't allow your opponent time to react, we don't know whether they're retiming those or not, but you would think that you can't wait [until] after one-third of the electorate has voted to drop that information."

Despite some limits on early voting since 2008, Gronke estimates that 35 percent of all votes will be cast before Election Day. "As voters choose this method, they tend to continue, and others flock to it," he says.

Gronke explains that early voting differs considerably by region, with Western states in particular embracing the trend.

"We really have three elections going on at once," says Gronke. "We have one big election but three kind of regional elections. In the West ... half the ballots overall come in by mail. ... Colorado will be 75 percent of the ballots by mail. California may be between 50 and 60 percent."

"Then, in the Southeast you have kind of a balanced system, where about one-third of the ballots are coming in 'no excuse' absentee, another third will be cast early in person ... and then the other third at the polling place" on Election Day.

"And then you have the Northeast, where people still stand on street corners. They wave signs with their coffee, and they vote on Election Day."

Gronke says changes in voting patterns have caused campaigns to adjust how and when they try to reach potential voters.

Early voters tend to be partisan, ideological, better educated and higher income, he says.

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