We conclude our Presidential Geography series, a one-by-one examination of each state’s political landscape and how it is changing, with Ohio, the Buckeye State. FiveThirtyEight spoke with Herb Asher and Paul A. Beck, both professors emeritus in Ohio State University’s department of political science.

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If the polls are correct, and President Obama wins a narrow Electoral College victory on Tuesday, the pivotal moment of the 2012 presidential race may have actually occurred in 2009. About two months after taking office, Mr. Obama set the terms of the government’s rescue of General Motors and Chrysler, a move that eventually helped to resurrect the American automobile industry, and, in turn, bolster the economy of the king of swing states: Ohio.

Historically, Ohio has been slightly Republican-leaning relative to the nation. But this year polls suggest that Ohio is slightly Democratic-leaning. That divergence — driven by the auto rescue and the state’s improved economy, local analysts said — may prove determinative. Ohio ranks first on FiveThirtyEight’s tipping point index. The model estimates there is roughly a 50 percent chance that the Buckeye State’s 18 electoral votes will carry the winning candidate past the 270 mark.

Ohio’s historic rightward lean has been slight — about two percentage points, on average, since 1948 — but consistent. In the 16 presidential elections from 1948 through 2008, Ohio was redder than the nation in 13. It was Democratic-leaning relative to the nation only in 1964, 1972 and 2004 (and in 2004, it leaned Democratic by less than half a percentage point).

But polls show Mr. Obama leading Mitt Romney in Ohio by about three percentage points, one point better than Mr. Obama’s projected national margin, according to the current FiveThirtyEight forecast. The auto rescue’s impact on Ohio’s political preferences, though modest, has been decisive.

“The auto rescue is popular in Ohio,” Mr. Beck said, and because the Buckeye State was only slightly Republican-leaning, a small shift appears to have tipped the state’s partisan balance.

Moreover, the auto rescue and Ohio’s steadily falling unemployment rate appear to have improved Mr. Obama’s standing with the very demographic group that Mr. Romney might have made inroads with: white working-class voters.

“White working-class voters in Ohio have been more supportive of Obama than white working-class voters nationwide,” Mr. Beck said.

Ohio’s economy has traditionally been driven by manufacturing. “Ohio led the industrial revolution 100 years ago,” Mr. Asher said, but in the latter half of the 20th century, globalization and the outsourcing of manufacturing jobs hurt the state’s economy.

Early in the 2012 presidential campaign, during the summer, the Obama campaign saturated Ohio television with advertisements highlighting Mr. Romney’s “Let Detroit Go Bankrupt” op-ed article in The Times as well as linking Mr. Romney to Bain Capital and linking Bain Capital to outsourcing, Mr. Asher said. For many Ohio voters, that effort helped undermine Mr. Romney’s contention that his business experience would benefit them if he reached the White House.

As a result, Mr. Beck said, many of the white working-class voters whom Mr. Romney might have appealed to now see him “as the kind of businessman who for many of them was the problem.”

Mr. Asher added, “The Obama campaign defined Mr. Romney, and that appeal gets reinforcement by the auto bailout.”

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The Democratic Party’s base of support in Ohio is in the northeast part of the state, a mix of African-American and blue-collar union voters in Cleveland, Canton, Akron and Youngstown. In 2008, Mr. Obama carried Ohio by just over 200,000 votes; he carried Cleveland’s Cuyahoga County by almost 250,000 votes. Northeastern Ohio is also the center of the state’s auto industry, in Cuyahoga and Lake Counties.

From Cleveland, Democratic support fades as you travel west along Lake Erie (though Toledo is reliably left-leaning) and southeast along Ohio’s border with Pennsylvania. Democrats have also made some gains in Ohio’s other major cities. Franklin County, which includes the state capital, Columbus, has trended Democratic, and Mr. Obama made gains in Dayton in 2008, as well.

In 2008, Mr. Obama also managed to flip Hamilton County, home to Cincinnati, which had long been one of the more Republican-leaning cities in the state.

Conversely, Democratic support has eroded in southeastern Ohio, which is part of Appalachia. The southeast is culturally conservative and economically depressed. Bill Clinton carried many of the counties there, but they have moved sharply toward the G.O.P. since then. Coal was a major economic driver in southeastern Ohio, and Mr. Romney has targeted voters in the area by attacking Mr. Obama’s energy policies. But the southeast is also lightly populated, contributing only about 10 percent of the statewide vote, Mr. Asher said.

Outside of the northeast and Columbus, Dayton and Cincinnati, Ohio is mostly Republican-leaning. Western Ohio, in particular, is ruby red and socially conservative. To carry the state, Mr. Romney will need to run up his margins in the suburban and exurban counties in southwestern Ohio around Cincinnati as well as the small towns along the state’s western border. In 2008, Senator John McCain carried those counties, but he did not get the turnout and margins that George W. Bush did in 2004.

The Bellwether: Stark County

Stark County, anchored by Canton, has been an almost perfect bellwether for the statewide vote in Ohio in the past three presidential elections. Canton tends to vote Democratic, but Stark County also has more rural areas that lean Republican. Stark County was one percentage point more Republican-leaning that Ohio over all in 2008 and 2000 and two points more Republican-leaning in 2004.

The Bottom Line

Mr. Obama is an 85 percent favorite to carry Ohio, according to the current FiveThirtyEight forecast. Not coincidentally, that almost exactly matches his odds of winning re-election, according to the model.

If not for the auto rescue, Ohio’s slight Republican lean would most likely have remained in effect. Unlike in other states, there are no major demographic trends affecting the state’s partisan balance.

“Ohio in terms of demographics is a fairly static state,” Mr. Beck said. “Our Hispanic population is too small to matter except at the margins.”

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If Ohio were, relative to the national popular vote, two percentage points Republican-leaning this election — its average over the last 60 years — the state would be a tossup. And if Mr. Romney were able to carry Ohio, he would have many more paths to the 270 electoral votes he needs to win the White House.

But if the polls are right, and the auto rescue and Ohio’s relatively healthy economy help Mr. Obama prevail in the Buckeye State, then it becomes difficult — though not impossible — for Mr. Romney to piece together a winning electoral map.