The state's geography is often divided regionally as the "Five Ohios." Here they are politically:
—NORTHEAST OHIO: Obama needs to pile up votes in the state's Democratic stronghold, anchored by Cleveland and the industrial towns of Youngstown and Akron.
But these areas, once home to dozens of steel, auto and tire factories, have lost population in recent years. A drop in turnout could be devastating for the president.
Cuyahoga County, the state's biggest, is also the most reliable for Democrats. The county includes Cleveland and working class suburbs. It gave Obama one of every six votes he won Ohio with four years ago. He had 70 percent of Cuyahoga's votes, 3 percentage points better than John Kerry in '04.
Romney needs to do well in Republican-leaning suburbs tucked between the cities. Unemployment numbers in the suburban areas are improving, and fall well below the state average of about 7 percent. One big question in this region and in northwest Ohio is whether voters will credit Obama and the federal bailout with revitalizing the auto industry, an important employer and economic driver.
—SOUTHWEST OHIO: The conservative suburban crescent around Cincinnati is the GOP's answer to Cuyahoga County; Butler, Warren and Clermont counties are Republican turf. Romney needs blowout totals. George W. Bush carried the region twice with 2-to-1 margins; he got 72 percent of Warren's vote in '04 as he clinched re-election.
Obama did 3 to 4 percentage points, some 23,000 votes more, better than Kerry in the counties and has a bigger presence this year. But Romney has held major rallies in the area to fire up the base.
Montgomery County, to the north, has been going Democratic in recent presidential elections, but Republicans think they are picking up strength and pro-Romney ads have highlighted Obama defense cuts in an area with the state's largest military base, Wright-Patterson Air Force Base.
To the south is Hamilton County. Obama was the first Democratic presidential candidate to win the Cincinnati-based county since Lyndon B. Johnson in 1964, and Republicans are anxious to win it back. They've had an aggressive ground game, and see added advantages: no competitive Democratic challengers in the two congressional districts, and it's the home base of Romney Ohio chairman Sen. Rob Portman.
—CENTRAL OHIO: Ohio's capital city of Columbus is friendly ground for Obama; it's led by a popular black mayor, with Democratic voters in trendy communities and inner-city neighborhoods. But Franklin County also has more Republican voters than any other county in Ohio, many with ties to its government, insurance and financial sectors.
Obama has tried to fuel enthusiasm with frequent rallies, building on strength at Ohio State University, among the nation's largest universities. Romney has campaigned often in affluent, heavily Republican suburbs that ring the city — such as Bexley, Powell and Westerville, where GOP Gov. John Kasich lives.
Republican-leaning counties stretch from Columbus northward to the mid-sized cities of Mansfield and Marion, middle-class communities.
—NORTHWEST OHIO: Ohio's conservative farm belt stretches up and down in the west through fields and small cities.
If conservatives in rural Ohio come out in huge numbers for Romney, they could be the difference. That's what happened eight years ago, when voters felt Bush shared their values. Voters in some of the conservative counties often put faith issues ahead of economic concerns.
But Obama isn't writing it off. Toledo, with its auto plants and heavy union influence, is a Democratic bastion. The auto bailout plays well in Toledo and may win votes in the surrounding rural areas where small companies make everything from car axles to seats.
—SOUTHEAST OHIO: This is Appalachia, stretching through small towns and hills along the Ohio River and then up along the West Virginia border in coal country.
It had been traditionally a swing area — Bill Clinton carried it twice, as did George W. Bush. But John McCain captured much of the region in 2008, despite losing Ohio. It's a predominantly white region, with Bible Belt conservatism on such issues as opposing abortion and gay marriage.
Romney has been playing on concerns about the future of the coal industry and thinks his campaign will improve on McCain's showing — and possibly turn a strip of three eastern Ohio counties — Belmont, Jefferson and Monroe — red for the first time since Richard Nixon won them in 1972.