DES MOINES – As eight bells rang from the clock tower of the Polk County Courthouse, the doors to the election office opened on Thursday morning and voters began casting the first ballots of the presidential race in this battleground state.

“It seems like we’ve been waiting for this day for a long time,” said Nancy Bobo, 60, who stood in line and voted for President Obama. “I’m just thrilled to get out here and vote as soon as I possibly could.”

Less than a week before the president’s first debate with Mitt Romney and a month before the closing arguments of a campaign traditionally would be made, a steady stream of voters walked into election offices across the state to cast their ballots. They will be joined by voters in Ohio next week, along with 30 states where some type of voting is already under way.

For millions of Americans, the election no longer is a fixed date on the November calendar. It is increasingly becoming an item on the fall checklist, a civic duty steeped in the convenience of everyday life. The development is profoundly influencing presidential races, with Election Day becoming Election Month for as much as 40 percent of the electorate this year.

The number of people casting early ballots nationally climbed to 31 percent in 2008 from 23 percent in 2004, 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 even higher in many battleground states.

In Florida, North Carolina, Colorado and Nevada, both campaigns believe that as much as 70 percent of the ballots will be cast before Election Day. And in Ohio, Wisconsin and Iowa, advisers to both campaigns said at least 30 percent of people are expected to vote early.

Iowa became the first swing state to allow voters to cast ballots in person this week, a provision of state election law that the Obama campaign has seized upon. In the opening hours of voting on Thursday, supporters of the president dominated the line here in the state’s biggest city. They were easily identifiable by their blue Obama 2012 stickers that declared, “Be the First!”

The bustling line slowed to a trickle after about two hours on Thursday morning. But the real burst of voting is yet to come, when the tens of thousands of absentee ballots that were previously requested by voters start arriving by mail, and through satellite voting locations that will be open across the state in places that were requested by the campaigns to maximize turnout.

The Iowa Secretary of State’s office said Democrats had a 5-to-1 advantage over Republicans in the numbers of absentee ballots requested statewide. Republicans said the numbers would level out over the next 40 days.

“We are going to close that gap in Iowa,” said Rick Wiley, political director of theRepublican National Committee. “Democrats in Iowa have a propensity to cannibalize their Election Day voters. What they’ve done is find people who would vote on Election Day anyway.”

While the early voting laws are not new in most states, election officials say campaigns are seeking to leverage them to a greater degree than in any previous presidential campaign.

“It has changed from one 13-hour Election Day to 40 days,” said Jamie Fitzgerald, the Polk County Auditor, who oversees elections here. “You are really seeing a lot more emphasis on early voting and ballots by mail.”

The Obama campaign invited Jason Alexander, the actor who played George Costanza on “Seinfeld,” to travel across Iowa to promote early-voting efforts on Thursday. The Romney campaign held a rally aimed at women, featuring SenatorKay Bailey Hutchison of Texas.

At the election office, as Ms. Bobo stood with other Obama supporters on the first day of early voting, she said she wondered where Mr. Romney’s supporters 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.”