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The first Republican debate on Thursday is being held in Cleveland — in the battleground state that could make it or break it for the GOP. Jason Miller/Getty Images

Ohio voters held the nation in suspense in 2004 as voting officials tallied the state's close results in the contest between Republican George W. Bush and Democrat John Kerry that November. Once the count was over, Ohio had decided for the country to give Bush a second term in the White House.

Ohio's key role in determining presidential contests is back in the spotlight Thursday as Republican 2016 contenders gathered in Cleveland for the first primary debate of this election season. Meanwhile, the GOP announced earlier this year that its national Republican convention, where one of the 17 candidates debating this week will likely be named the nominee, will also be held in Cleveland. Ohio is a crucial win for any White House hopeful because of its diverse mix of elderly, black, white, Hispanic, rural and urban voters who have combined in recent elections to create a key battleground for any campaign. Even without going into the numbers associated with the Electoral College system that determines who wins the election, there’s a clear historical precedent: No Republican has ever lost Ohio and won the presidency.

"It’s a classic swing state and it’s going to be a tremendous battleground in '16," said Stuart Stevens, a top strategist for Mitt Romney’s 2012 presidential campaign. Stevens’ consulting firm currently works with New Jersey Gov. Chris Christie’s campaign. “If you don’t win Ohio, it’s really hard to win the electoral map if you're a Republican," Stevens said. "It’s possible, but it’s very difficult. It’s been such a tight state in the past.”

Current Polling of Candidates in GOP Debate | InsideGov

Ohio is a key swing state largely because in most states presidential elections aren't very competitive. Across the nation, the Electoral College voting structure gives Democrats a clear advantage. There are 242 votes almost certain to end up in the hands of Democrats and just 206 for Republicans, according to Ford O’Connell, a Republican strategist who worked on John McCain’s 2008 presidential campaign. To win an absolute majority, 270 votes are needed. There are only about seven states that don't lean strongly toward one political party, and Ohio is the second biggest after Florida.

“If you’re a Republican, you have to win Florida and Ohio,” O’Connell said. “If you’re a Democrat, you only have to win Ohio.”

In 2012, President Barack Obama carried Ohio with 3 percent of the vote. In 2008, he won it by just under 5 percent. In 2004, Bush carried the state by slightly more than 2 percent of the vote, a race that was close enough that it drew out the final decision of the election into the next day.

Democrats argue Ohio is poised to help them win the White House once again. Former secretary of state and Democratic presidential candidate Hillary Clinton was expected to travel to Cleveland in late August to meet with voters.

“We have complete, 100 percent confidence that we’re going to [win Ohio] again,” said Kirstin Alvanitakis, the communications director for the Ohio Democratic Party.

Republicans are also battling hard for Ohio. The Republican National Committee planned to send some 200 more field workers to the state this year. Part of the GOP's strategy is to simply raise the profile of Republicans in Ohio as it's a fairly expensive media market because it is so large and competitive.

Ohio has about 185,000 Hispanic eligible voters. Roughly 12 percent of Ohioans are black, and 83 percent are white, a key constituency for Republicans. At least 15 percent of Ohio workers are represented by unions, compared to the national average of 13 percent. Ohio also has a growing population of elderly voters, another key vote for the GOP.

“The fastest-growing group in the country is older voters, which we tend to forget," Stevens said.

So, what do Ohio voters care about?

“You can bet that one is the economy,” O’Connell said. They are also focused on foreign policy and terrorism. “In a lot of ways, they’re a lot more like the national Republican voter,” O’Connell said.