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, whose 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," Sen. Rob Portman of Ohio, the state chairman of Mitt Romney’s campaign, said at a campaign rally last week. "If you go down and vote, that frees you up to help others on Election Day."

When President Barack 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. 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.

The every-day-is-Election-Day effects of early voting have 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 instead of one frantic day.

More than 14.3 million people have voted so far, according to Michael P. McDonald, an associate professor of government and politics 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.

"Both registered Democrats and registered Republicans are voting at clips that are outpacing their 2008 levels," he said in an interview.

Both parties have been frantically spinning cherry-picked statistics to paint their early-vote operations as a success. While the true measure of their success will not be known until all the ballots are cast and counted, a look at who has voted early so far and where they live does give some meaningful indications of how the early vote is going in some of the key swing states where the election will be decided.

Democrats appear to have an advantage with early voting in several of them. Iowa Democrats had cast nearly 59,000 more early votes than Iowa Republicans through the end of last week. A state law there allows campaigns to petition election officials to open temporary voting locations, which have popped up in Mexican restaurants, evangelical churches and libraries. When Obama visited Cornell College in Mount Vernon the day after his second debate, a voting site was set up just across campus, with giant chalk-drawn arrows on sidewalks to guide students to cast their ballots. That day 433 people voted, according to Tim Box, the deputy commissioner of elections in Iowa’s Linn County.

More early votes have been recorded this year in Nevada than were four years ago, and more than 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.

But in some states, there are indications that Republicans are narrowing their early vote deficits. 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. It is a wide margin, but the question is whether it will be wide enough: Obama won the early vote in North Carolina by an even wider margin four years ago, McDonald noted.

And it was that wide margin that helped Obama win the state — the early vote propelled him to victory even though he received fewer votes than Sen. John McCain on Election Day.

Republicans have an edge in casting early votes in Colorado, where they have cast nearly 20,000 more than Democrats. Since Republicans in many states are more likely to wait until Election Day to cast their ballots, the party’s mission in many places is simply to whittle away at the Democratic advantage in early voting — so that Democrats will have a smaller cushion of votes going into Election Day.

Some Republicans argue that the Democrats are effectively cannibalizing their Election Day turnout, saying that many of their early voters appear to be frequent voters who would vote anyway.

"Republicans have been focused on increasing turnout among those Romney supporters who are less likely to vote and banking those votes during the early vote period," a blog post on the Republican National Committee’s website said.

Several states controlled by Republicans cut back early voting hours this year. After officials in Ohio announced plans to eliminate early voting in the days before the election, with an exception for members of the military, the Obama campaign sued. They prevailed in the case, which made it all the way to the Supreme Court. Republican officials in Florida scaled back their early voting hours this year, too, eliminating voting on the Sunday before Election Day, when many black churches conducted "souls to the polls" voting drives in the past.

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 statistics provided by the state. 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 independent voters — whose votes could prove decisive.

In Ohio, party affiliation is difficult to gauge, because the state does not register voters by party; the only indication of party affiliation is which primary they last voted in. The state’s decision to send absentee ballot request forms to all voters this year for the first time led to an increase in absentee ballot requests from rural, traditionally Republican counties, said McDonald of George Mason University, but also in urban counties. In Cuyahoga County, the Democratic stronghold around Cleveland, more early votes have been tallied so far this year than there were four years ago. A Time magazine poll released last week found that respondents who said that they had already voted favored Obama over Romney by a margin of 2-to-1.

Bill Dorsey, 68, a retired teacher in Ohio, cast his ballot for Obama last week at the Early Voting Center of Franklin County, on the north side of Columbus in a former Kohl’s department store that closed last year.

"If I drop dead before Election Day," said Dorsey, "my vote still counts."

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