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DES MOINES — A stream of voters arrived at election offices across Iowa to cast their ballots. Waves of absentee ballots have started landing in mailboxes in 30 other states. And more than a month before what the calendar says is Election Day, President Obama began delivering his closing argument to voters.

The rise of early voting, which got under way here on Thursday, is changing the rhythms of how Americans elect their presidents. The president is not as fixated on winning more votes than Mitt Romney on Election Day, but on executing a plan to accrue more votes over the next 40 days.

For millions of Americans, the election is no longer on a fixed date. It is increasingly becoming another item on the fall checklist, a civic duty steeped in the convenience of everyday life. The development is reshaping campaigns, with Election Day becoming Election Month for as much as 40 percent of the electorate this year, including voters in the vital swing states of Ohio, Florida, Colorado and others.

“It has made the October surprises way less relevant,” said Jim Messina, the campaign manager for Mr. Obama, who has built the president’s re-election strategy around the growing trend of voting early. One example: a two-minute ad that began running Thursday summing up Mr. Obama’s case for re-election. “In a close election, you can increase your number of voters in a very important way.”

The president opened his campaign speeches this week with a pitch for early voting, imploring Ohio voters, “I need you to start voting six days from now.” It was a not-so-subtle effort to bottle his early success and capitalize on what several polls find is an edge over Mr. Romney in swing states, which could shrink as the remainder of the race unfolds, with the first debate next Wednesday.

As the bell tolled eight from the clock tower of the Polk County Courthouse on Thursday, signaling the moment when the polls here would open, a line stretched down the street from the election office. A subject of conversation among those waiting was a statistic from 2008: Mr. Obama received fewer votes than Senator John McCain on Election Day in Iowa and some other states, but Mr. Obama won those states because his plan was built around a month of voting, not a day.

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The rise of early voting, which is allowed with few restrictions in 32 states and the District of Columbia, has opened a new front in efforts to maximize turnout and find voters through exhaustive micro-targeting. An open question, though, is whether making voting more convenient will mean that more people actually take part in the presidential election.

An Iowa law, which national election observers say is the only one of its kind in the country, allows a campaign to gather 100 signatures and petition election officials to create a temporary voting location aimed at serving a particular constituency.

Here in Des Moines, Democrats requested that a voting site be opened Oct. 20 at La Tapatia Tienda Mexicana, a restaurant. Republicans requested a site be opened on the same day at Johnston Evangelical Free Church. Election officials granted both requests, along with those for voting sites at libraries, grocery stores and community centers.

When Michelle Obama visits the University of Northern Iowa on Friday, her chief task will not be simply to deliver a speech. She will ask supporters to cast their ballots on the spot, a few steps away at a voting site requested by the campaign and approved by election officials.

While some people will vote in person, even more will do so by mail. The Iowa Secretary of State’s office said Democrats had a 5-to-1 edge over Republicans in the numbers of absentee ballots requested statewide — largely because of efforts by the Obama campaign — but Republicans said the numbers would level out over the next five weeks.

“We are going to close that gap in Iowa,” said Rick Wiley, political director of the Republican National Committee, which is overseeing early-voting efforts as part of its national field program. He added, “In years past, we were slow to embrace it, but it’s foolish not to.”

The proportion of people nationwide casting early ballots climbed from 23 percent in 2004 to 31 percent in 2008, according to Michael McDonald, who studies early voting at George Mason University. This year, party strategists estimate that up to 40 percent of voters will cast ballots before Nov. 6, but the proportion is far higher in many battleground states.

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In Florida, North Carolina, Colorado and Nevada, advisers to both campaigns say as many as 70 percent of ballots will be cast before Nov. 6. And in Ohio, Wisconsin and Iowa, the campaigns estimate at least 30 percent of people will vote early. Virginia and New Hampshire are the only battleground states without widespread, no-excuse early voting.

Republican officials in several states acknowledge that the Obama campaign may start with a slight advantage in early voting because Democrats have grown more accustomed to casting their ballots early. To level the playing field, the Republican secretary of state in Ohio sent absentee ballot requests to every registered voter in the state.

Tom Zawistowski, president of the Ohio Liberty Coalition, a group affiliated with the Tea Party, sent a message to encourage members to consider voting early. He wrote, “I know we do not like absentee voting or early voting at all, but it is a key part of our election equation now and we need to understand how to use it to our advantage just like the other side does.”

Here in Des Moines, the line slowed to a trickle after a few hours on Thursday morning, but the real burst of voting will come when absentee ballots start arriving by mail as early as Friday in voters’ mailboxes. The Obama campaign is deploying hundreds of field organizers and volunteers this weekend to “chase ballots,” or return envelopes to county election offices. The Republican Party here is sending a mailing to all of its voters, urging them to request an absentee ballot and vote before Election Day.

As Nancy Bobo, 60, stood with other Obama supporters, she wondered aloud where the supporters of Mr. Romney were.

“I don’t see them,” she said with a smile. “But we’re not taking anything for granted. We still have 40 days to go. You never know; things can change on a dime.”

But in the northwest corner of Iowa, more than 200 miles away in the town of Orange City, Gert Kooi, 76, was among those voting for Mr. Romney on Thursday.

“I voted today because we might not be here on Election Day, and my mind is long made up,” Ms. Kooi said in an interview. She added, “We just don’t care for Obama here.”