Nov. 6 is Election Day, but many voters have already cast their ballot for President Obama or Mitt Romney.

Dozens of states have launched early voting, including a number of crucial battlegrounds such as Ohio. About 30 percent of the votes cast in the 2008 presidential race were submitted before Election Day, and experts said an even greater number will do so this year.

Democrats bested Republicans in early voting four years ago, and part of Obama's strategy this time around is to generate a heavy early voter turnout that could offset any late gains by Romney. Voters are locking in their choices just as polls are showing Romney closing the gap with Obama and, in some cases, taking the lead for the first time following strong debate showings.

In Iowa, Democrats are outpacing Republicans by roughly 25 percentage points in early voting. Democrats are also turning out in greater numbers in Ohio, one of the most hotly contested states.

"The Obama ground efforts continue to dominate what Romney has been able to do," Democratic pollster Margie Omero said. "There's an enthusiasm with the base that exists for Obama that has never existed for Romney."

Thirty-two states and the District allow all voters to cast their ballot early. In 21 others, including Virginia, voters can cast an absentee ballot only under certain conditions, such as absence for military service.

Republicans insist they're not conceding the early vote to Obama. Romney's aides contend they are closing the gap with the president in early voting in Iowa and Ohio and have a lead in both Florida and North Carolina among absentee voters.

In-person, early voting began in North Carolina on Thursday, and Romney dispatched former Pennsylvania U.S. Sen. Rick Santorum, a conservative who challenged Romney for the GOP presidential nomination, to the state to rally voters. GOP operatives also will soon descend on Colorado, Nevada, Florida and Wisconsin to aid early-voting efforts.

Those moves could be rewarded, some analysts said.

"I don't think Romney can expect to win the early vote nationally, but certainly he can limit the damage," said Michael McDonald, a political scientist at George Mason University and expert on early voting. "We would certainly expect the Romney campaign to do better than the McCain campaign. The Republicans were unprepared in 2008."

This year, both Obama and Romney are holding rallies to highlight the importance of early voting in hopes of banking as many votes as possible so they can focus on a shrinking pool of undecided voters heading into the final weeks of the race.

But some cautioned against relying too much on early voting as a measure of how a candidate will do on Nov. 6. In North Carolina four years ago, Obama enjoyed a 305,000-vote advantage in early voting but eventually won the state by just 14,000 votes.

"In my experience, it's just the hard-core partisans who tend to vote early," said Republican pollster Whit Ayres. "They'd be unlikely to change their vote. I'm not persuaded that leading in the early vote is a major advantage."