(CBS News) Election Day is seven weeks away, but before Nov. 6, around 44 million people are likely to have already cast their votes.
Early voting, which starts in South Dakota and Idaho today, has grown more common with each presidential election over the last 12 years. It's a shift that makes voting more convenient, accessible and gives early indicators as to which side has an edge when it comes to enthusiasm and turnout efforts. Early voting only boosts overall voter turnout marginally, but in states like Ohio and Florida -- where lawsuits over early voting are pending -- a marginal boost in turnout could make all the difference.
In-person early voting opens today in South Dakota and in some counties in Idaho, though ballots are already coming in from a handful of other states: Kentucky, Indiana and the battleground states of North Carolina and Wisconsin.
In those four states, voters can already submit absentee or mail-in ballots. However, for a more accurate understanding of when the campaigns can drive voters to the ballot box en masse, CBS News considers early voting to start when a state has opened drop-off centers for ballots or in-person voting sites.
Since 2002, after the passage of the Help America Vote Act, many states adopted rules allowing large numbers of people to vote before Election Day. Currently, 34 states (including 10 battleground states) plus the District of Columbia offer early voting.
Just 16 percent of votes in the 2000 election were cast early, but that figure rose to 22 percent in 2004 and 33 percent in 2008. This year, the CBS News polling unit predicts at least one third of ballots -- or around 44 million -- will be cast before Election Day, though that figure could reach as high as 40 percent. In battleground states, early votes could account for anywhere from an estimated 5 percent of the vote (as in Pennsylvania) to an estimated 80 percent (in Colorado).
According to Paul Gronke, director of the Early Voting Information Center at Reed College in Portland, Ore., "People who vote early are people who've made their minds up."
Given what polls are showing, that means early voters could be more crucial to the candidates than ever before: Just 4 percent of likely voters in the last nationalsaid they were undecided. Among those who back a candidate, more than eight in 10 said their minds are made up.
It may be no surprise, then, that the candidates are already nudging their supporters in early voting states to mobilize.
"You can start moving in that direction in 10 days when early voting starts, so go, go now, go now, get engaged," Vice President Joe Biden told voters in Burlington, Iowa on Monday. "Get engaged, but you can start to vote early, start to vote early in the next 10 days."
Early in-person voting in the battleground state of Iowa starts next week, on Sept. 27. Republican vice presidential candidate Paul Ryan was also in the state Monday and similarly told the crowd, "I gotta tell ya, Iowa. Early voting starts in a week. We need you to get out and vote. Because states like Iowa could very well determine the future of this country."
With weeks to cast their ballots, people in early voting states are inundated by advertisements in the mail and on television, phone calls and knocks on the door from the campaigns -- at least until the day they cast their ballot. Early voting states will regularly release lists of citizens who have already voted, allowing the campaigns to cross those voters off their lists and move on to the next person.
"Campaigns are supposed to be all about Election Day," Gronke explains. But in early voting states, "it's like Groundhog Day, doing the same thing over weeks."
Locking in votes early can have its advantages. The campaigns can tailor their tactics for voters who have yet to cast ballots. On top of that, it buffers candidates from unexpected campaign missteps or late-breaking news that could turn off voters. In 2000, for instance, voters learned just days before Election Day that George W. Bush had been arrested in 1976 for driving under the influence.
Voters who cast early ballots, according to Gronke, tend to be better educated, older, fall into higher income brackets and are more often white. But in 2008, African-Americans voted early at an unprecedented rate to support Barack Obama. Many of them voted on the Sunday before Election Day, participating in get-out-the-vote programs known as "souls to the polls," which take voters directly from church to polling places.
Given the demographic trends visible, attempts to restrict early voting have created an uproar. The Obama campaign sued the state of Ohio over its new state law that would have shut down early voting after 6 p.m. on Friday, Nov. 2 until Election Day the following Tuesday -- except for military personnel. A federal judge overturned the law, ruling it unconstitutional because it created two classes of voters and gave one -- military personnel -- special treatment. Ohio is appealing the ruling.
Similarly in Florida, lawmakers reduced the number of early voting days from 14 to eight, but a federal court said the change unfairly hurt minority voters. In a compromise with the federal government, the state agreed to extend voting hours on the eight days of early voting. However, Democratic Rep. Corrine Brown is also suing the state and argues that eliminating voting on the Sunday before Election Day unfairly hurts black voters.
While the campaigns and state officials spar over early voting days, Gronke said it may not have the impact some would expect. Evidence shows that offering early voting only increases turnout by 2 to 4 percent in federal elections -- lower than researchers anticipated, Gronke said.
And in past elections, early voting hasn't always been an indicator of the ultimate results. In 2004, Democrat John Kerry won more early votes in Iowa but still lost the state by 10,000 votes once every vote was counted.
Still, that slight boost in turnout could make all the difference in a state like Ohio or Florida, particularly this year.
"With the election so close, this is like total warfare, and this just another front," Gronke said.
Anthony Salvanto and Jennifer DePinto contributed to this report.