By Eric Johnson, Kim Palmer

CINCINNATI, Ohio (Reuters) - First lady Michelle Obama rallied supporters to back her husband as early voting began on Tuesday in the key electoral swing state of Ohio where the Democrats hope to take advantage of a lead in opinion polls.

“Are we going to just sit back and watch everything we worked for and fought for just slip away?” she asked a boisterous crowd of 6,800 in downtown Cincinnati.

With President Barack Obama focused on Wednesday’s debate against Republican challenger Mitt Romney, the first lady led the Democrats’ charge in Ohio, directing supporters to march to a local election office and cast ballots 35 days before the main voting day of November 6.

“All of our hard work, all the progress that we made, it is all on the line this November,” she said. “Here in Ohio, it is already Election Day.”

Urging voters to cast their ballots early in person or through mailed-in absentee forms has become a major part of the Obama campaign’s strategy in Ohio and other swing states.

People who vote early tend to rally friends and relatives among groups that are less frequent voters such as the young, low-income people and ethnic minorities. Those groups are generally more likely to vote Democrat.

The Obama campaign is peppering less enthusiastic supporters with phone calls, home visits, and direct mail pleas.

In 2008, roughly 30 percent of all ballots cast were early. Of those, nearly 60 percent favored Obama.

The Obama campaign seeks to persuade at least half of likely supporters to cast early ballots in Ohio and elsewhere, a campaign official said

Voters backing both campaigns streamed into polling locations on Tuesday across 88 counties and requested more than 920,000 absentee ballots, election officials said.

After months essentially tied with Romney in Ohio, Obama is now leading by roughly 5 percentage points in Ohio, according to aggregator RealClearPolitics.

Some 7.8 million Ohioans are registered to vote in the state, compared with 8.2 million at this point in 2008, according to an election official.

Daunte Thomas, 18, walked with fellow students from Cleveland State University to cast their first votes for Obama.

“I didn’t know today was the first day to vote before but we got (campaign) emails and there were people ... this week on campus and they told us,” Thomas said.

HURDLES FOR OBAMA

The Obama team’s Ohio infrastructure faces hurdles.

An appeals court could overturn a judge’s decision to keep polling places open the three days before Election Day, a window during which some 105,000 ballots were cast in 2008, a campaign spokeswoman said.

But perhaps its biggest obstacle is the Romney team, which has some 40 offices in the state and is fortified by powerful “Super PAC” allies paying for negative television advertisements that hammered Obama’s stewardship of a frail economic recovery and his healthcare law.

Romney’s team will have knocked on one million doors by the end of the week and it has made three million phone calls since May - more than in any other swing state, Romney officials said.

An early voting event for the former Massachusetts governor on Tuesday in downtown Cincinnati drew roughly 45 supporters, among them was John McHugh, a 60-year-old former auto worker.

McHugh, who joined other volunteers distributing campaign fliers at the voting location, said Obama is “tone deaf” on the economy and “a failure” on lowering joblessness.

“I don’t think he is working towards the same America I am.”