But numbers show Democrats are taking early leads in some key battleground states, like Iowa and Florida.
With more than one in three votes likely to be cast before Election Day this year, Republicans are stepping up their efforts to chip away at what has been a Democratic advantage in early voting in key battlegrounds like Ohio and North Carolina.
In Ohio, where 18 electoral votes are at the center of the presidential race, more than 1 million votes have already been cast, highlighting a change in the political rhythm that has led Republicans to begin to embrace the belief long held by Democrats that early voting can be used to increase turnout, not just to shift votes from one day to another.
"Some Republicans don't like to vote early, they like to go on Election Day. I understand that," said Sen. Rob Portman of Ohio, state chairman of Mitt Romney's campaign, at a campaign rally last week. "If you go down and vote, that frees you up to help others on Election Day."
When President Obama flew home to Chicago last week to cast his ballot, he became one of the millions of Americans who have already voted -- a flood of early votes that is reshaping how both campaigns operate.
The early vote gave Obama his margin of victory in several key states four years ago, and Democrats are trying to maintain that advantage this year by banking as many early votes as they can. But Republicans are trying to dampen any early Democratic edge by making a bigger organizational push than they did in the last election.
The Sandy factor
Hurricane Sandy has introduced more uncertainty into the mix: It forced the closures of early voting sites in North Carolina and some in Virginia on Monday, and the storm could curtail early voting hours in other key states as it moves inland.
Early voting has transformed modern campaigning, from the splashy Bruce Springsteen concerts the Obama campaign organized this month to mobilize supporters to the polls, to the less-glamorous databases that the campaigns keep to track potential early voters, as their get-out-the-vote operations have stretched into weeks.
Nearly 15 million people have voted so far, according to Michael P. McDonald, an associate professor at George Mason University who keeps tabs on early voting. He said that the pace of early voting this year suggested that 35 percent or more of all votes could be cast before Election Day, surpassing the previous record in 2008 when 30 percent voted early.
Democrats appear to have an advantage with early voting in several states.
Iowa Democrats had cast nearly 59,000 more early votes than Iowa Republicans through the end of last week.
More early votes have been recorded this year in Nevada than four years ago, and about 35,000 more early votes have been cast by Nevada Democrats than by Republicans, giving Democrats a 46 percent to 36 percent lead in ballots cast in person by early voters.
In North Carolina, about half of the 1.5 million votes received so far were cast by Democrats, giving them an advantage of nearly 20 percentage points above Republicans.
Republicans have an edge in casting early votes in Colorado, where they have cast nearly 20,000 more than Democrats.
Some 1.9 Floridians have already voted. More than half a million people there have gone to the polls since the state began in-person early voting over the weekend, and Democrats cast more than 78,000 more votes than Republicans, according to state statistics. The first weekend of in-person voting erased the 61,000-vote edge that Republicans had run up with absentee ballots, most of which were mailed in. But in Florida more than 300,000 early and absentee votes were cast by independents -- whose votes could prove decisive.