CINCINNATI — When it comes to presidential politics, Ohio has become the king of swing.

Ohio is a perennial battleground state, and the battles every four years are long and expensive. With more than a month left before the election, the two presidential campaigns and their outside supporters already had spent more than $110 million on TV advertising in Ohio, a sixth of all national spending to that point.

President Barack Obama has been to Ohio this year more times than any other state except Virginia, and he chose Ohio State University in Columbus for his first re-election campaign rally in May. Mitt Romney — who started campaigning for Ohio’s Republican primary last year — has been here even more often, with Democratic Vice President Joe Biden and GOP running mate Paul Ryan also making frequent visits.

The stakes are high. No Republican has won the White House without carrying Ohio, and most projections say Romney needs Ohio and its 18 electoral votes to win this year. Meanwhile, repeating his 2008 victory in Ohio would simplify Obama’s electoral math, and also keep him on the side of history — the last president elected without winning Ohio was Democrat John F. Kennedy in 1960.

Ohio’s importance “is reinforced in their allocation of resources,” said Gene Beaupre, Xavier University political scientist. “They start early and they come here and fight it out, often down to the last week.”

Recent elections have been close: Obama won with 51.5 percent of the vote, after George W. Bush won with 50.8 percent of the vote in 2004 and barely 50 percent in 2000.

Polls indicated Obama had opened a lead over Romney heading into the final weeks, but both sides expect the race to remain hotly contested and are focused on get-out-the-vote efforts; walking neighborhoods and calling voters from nightly phone banks. Both sides say they have more organization in place than the campaigns did in 2008 at this stage and are rolling along at high rates on voter contacts.

“Living in a battleground state, I do feel like I have a lot more responsibility to do something,” said Libby Rupp, 47, of the northern Cincinnati suburb of Mason. She has been spending weekend afternoons doing door-to-door canvassing for the Obama campaign.

“I know how important Ohio is in this election, and that’s why I’m out here working,” said Beth Hamad, 21, of the eastern Cincinnati suburb of Anderson Township. She goes out three to four times a week canvassing for the Romney campaign.

Scott Jennings, state director for the Romney campaign, said the Republican side is well-positioned for the final push in Ohio. While the Obama campaign had the early benefit from the structure left in from its 2008 campaign in Ohio, Jennings said the Romney campaign has been building momentum.

“Each week we’ve done more, done more, done more,” Jennings said. “We’ve got a whole heck of a lot of people. I am pleased as punch with this thing.”

Jennings has swing-state experience from helping direct Bush’s successful effort to win New Mexico in 2004. On the Obama side in Ohio, senior adviser Aaron Pickrell, who was state director in 2008 and also has worked for former Democratic Ohio Gov. Ted Strickland’s statewide campaigns, said the campaign has been organized and active for a much longer time in the state than it had been in 2008, and is doing more to pick up votes in Republican-leaning areas besides in its strongest bases led by the Cleveland area.

“We’re going to fight for every vote ... in every corner of the state,” Pickrell said.

Ohio’s recovering economy appears to have helped Obama, even though Republicans say GOP Gov. John Kasich deserves credit. The president, Biden and the rest of the campaign repeatedly point to his support for the U.S. auto industry bailout, and the industry’s importance to Ohio. Some good news came in August, when General Motors announced it would invest $220 million in northeast Ohio plants to produce the new version of the Chevrolet Cruze, and Chrysler plans to add workers at an assembly plant in Toledo.

Ohio’s unemployment rate has dropped more than 3 points since its recession peak, and at 7.2 percent in August was nearly a point below the nation’s rate for the same period.

Romney and his campaign contend that too many Ohioans are still struggling. They have portrayed the Obama administration as hostile to coal, still a big employer in eastern Ohio; weak on China trade that costs Ohio jobs, and as making defense cuts that hurt bases and plants across the state.

The Romney campaign has seen benefits from Ohio campaign chairman Sen. Rob Portman, who has called on his own statewide network while helping Romney prepare for the presidential debates, seeking out friends such as country singer Rodney Atkins and NFL Hall of Famer Anthony Munoz to help. Wisconsin Congressman Ryan, chosen by Romney over Portman and other VP contenders, has a link to Ohio as a Miami University graduate.

Both sides can look to the most recent statewide elections in 2011 for encouragement.

The Obama campaign thinks it has gained support from the union workers who mobilized last November to overwhelmingly repeal a Republican-led effort to restrict collective bargaining for teacher, police, firefighters and other public employees. But on the same vote, Ohioans swung against Obama, soundly rejecting his signature health care legislation.