Next week, polls open for early voters in Iowa, a swing state, while Ohio, another critical state, begins its early-voter season, ABC News reported.
North Carolina absentee ballots were mailed Sept. 7, the day after President Obama accepted his party's nomination at the Democratic National Convention in Charlotte. In 2008, 56 percent of the total vote in the Tar Heel state was cast early or by absentee ballot.
"Since 2000 the rate of increase has been 50 percent in every presidential election" Paul Gronke, Director of the Early Voting Center at Reed College, told ABC News. "In 2000 it was about 15 percent; in 2004 it was about 22 percent; in 2008 it was about 33 percent."
If trends seen in 2008 hold for 2012, key swing states such as Florida, Nevada and Colorado could see at least half of their total votes cast by Election Day, ABC News said.
In 2008, early voting accounted for 58 percent of the vote in Nevada and 57.3 percent in Florida, ABC News said. In Colorado, 63 percent voted by mail-in ballot. Secretaries of state in Ohio and Iowa said 30 percent and 36 percent, respectively, of the voters cast their ballots early.
Early voters generally tend to be more partisan, Gronke told ABC News.
"The broad-brush strokes is that people who vote early tend to have their mind made up, so they tend to be a bit more partisan, a bit more ideological, a bit more informed about politics," he said.