CLEVELAND – Donald Trump has already won big with Republicans. On Thursday night, he accepted the party's nomination for president at the national convention held in Ohio's second-largest city. But in some areas of this swing state, he's winning big among Democrats too.

Southeast of Cleveland, near the Pennsylvania border, sits Mahoning County, once famous for its booming steel industry in Youngstown. Now, the county is suffering from the manufacturing decline felt in much of the Rust Belt.

Trump's candidacy has apparently tapped into the economic frustration felt in the region. In Ohio's March primary, 6,171 Democrats in Mahoning County cast their ballot for a Republican candidate, according to CNN, formally changing their party affiliation from Democrat to Republican.

Mark Munroe, chairman of the Mahoning County GOP, witnessed the phenomenon firsthand. In the months leading up to the primary, he said his office was flooded with calls by Democrats and independents seeking to change their party affiliation.

"By and large, I was talking to people who were excited about the Trump candidacy," he said.

By primary day, Trump's impact was clear.

"On election night, I saw how many Republican ballots were cast," Munroe said. "The number was like 34,000. We only had 14,000 Republicans in the whole county."

More than 6,000 Democrats and 14,000 previously unaffiliated voters had cast their primary ballot for a Republican, Munroe said, while fewer than 200 Republicans had changed their party affiliation to Democrat.

Dave Betras, chairman of the Mahoning Valley Democratic Party, noticed the trend too. He was forced to remove 18 precinct committee members after they cast their votes for the other party.

Guy Coviello, the vice president of government affairs at Youngstown Warren Regional Chamber, thinks the voter crossover stems from Trump's ability to speak to the desires of the Mahoning County electorate.

"A lot of the people who work in the manufacturing fields have had some ill feelings about free trade agreements," he said. "What they hear Donald Trump saying about free trade agreements is very appealing to them."

Paul Sracic, the chair of the Department of Politics and International Relations at Youngstown State University, echoes the sentiment.

"[Trump] holds a very strong position against free trade agreements, things like NAFTA," he said. "That is something that has been traditionally opposed by voters in Mahoning Valley. I often used to joke with reporters that only Osama bin Laden was more unpopular than NAFTA in the Mahoning Valley."

Trump capitalized on his strong stance against free trade at the Republican National Convention Thursday by reiterating his promise to the American blue collar workers in his speech.

"I have visited the laid-off factory workers and the communities crushed by our horrible and unfair trade deals," he said. "Our steelworkers and our miners are going back to work again."

But Betras isn't convinced.

"Back in the 80s, even before NAFTA was passed, they started tearing down our steel mills," he said. "Where once great and mighty steel mills used to be with smoke billowing out of their smokestacks is just brown fields now. There are no steel mills left here. He's just bullshitting the people."

Munroe credits Trump's personality as a driving force behind the trend in this year's primary.

"He developed the voice that was louder than everybody's and that's what the voters heard," he said. "Trump became a steamroller that just plowed everything down in his path. His campaign caught fire, caught the attention of the voters, and he took off like a rocket."

Trump may have caught the attention of these party-switching voters, but it's not clear whether the crossover will last.

"It's a Trump phenomenon, I don't think there's any doubt about that," Coviello said. "[Poll workers talked] about people saying 'I want the Trump ballot' instead of saying 'I want the Republican ballot.' There is no Trump ballot."

Reach Contributor Rachel Chiu here, or follow her on Twitter.