Updated

With more and more Americans casting their votes for president well before Election Day, early voting could be a key factor in deciding one of the most closely watched battlegrounds of the 2012 race, Ohio, which starts early voting Tuesday.

As an expansion of mail-in absentee ballots, laws in more than half the states allow voters to cast ballots in person well before Nov. 6. And the Obama campaign is counting on a big early turnout in key swing states.

The process tends to benefit Democrats, and the Obama campaign has made it part of its ground organization: informing their supporters where to vote, explaining options for how to vote and promoting opportunities to cast votes early.

Nevada, where Obama is holding his debate prep this week, was one of the first states to adopt the policy. It was started to increase participation in the voting process among people who might not be free to show up in person on Election Day. It also decreases lines at polling sites on the day of the election. Thirty-five states and the District of Columbia allow some form of in-person early voting.

In Iowa, the process began last week with much enthusiasm. More than 25,000 voters have already excercised their right. Here in Nevada, the early-voting window in shorter, but the Obama team sees the Silver State as key to their victory next month.

"It's important for us to be spending time here, while the president is here preparing for the debate Wednesday,” campaign spokeswoman Jen Psaki told reporters. She said that in addition to debate prep, it was important for the president to “spend some time in the state before early voting starts."

But in Ohio, the quintessential bellwether state and a must-win for both camps, the Obama campaign is ahead in the polls by 8 percentage points as early voting commences. The campaign canceled a stop in the Buckeye State later this week to focus on Wisconsin, where polls are tighter and where early voting also starts in a couple weeks. But Psaki insists the Obama team is not putting Ohio in the win category just yet.

“That's a key part of our strategy and we know what the polls have said, but we absolutely expect they're going to tighten in Ohio, which is why we're going to spend more time there, why we continue to be up with ads, why we're very focused on our ground game," she said.

One state where Mitt Romney might have the early voting advantage is in North Carolina, historically a red state that swung blue for Obama four years ago. The early-voting focus there is on mail-in absentee ballots, many of them military, which tend to vote Republican.

In 2008, a third of all votes cast were done so before election day, with 58 percent of those early votes supporting the president. Experts estimate early voting will increase in 2012. As many as 40 percent could cast ballots before Nov. 6.