Days after protests forced the Republican front-runner to cancel a rally in Chicago, Donald Trump has suffered a significant defeat in Ohio's key primary.

John Kasich, the moderate governor of Ohio, managed to pull ahead in the neck-and-neck race on Tuesday, denying Trump all 66 delegates under the state's winner-take-all rules.

The billionaire reality television star's loss in Ohio slows, although it does not halt, his steady march toward the Republican nomination.

"Losing Ohio means he'll have to deal with Kasich for a little bit longer, but it doesn't end the process," said David Nevin, a US politics expert at the University of Cincinnati.

"The way the map and the way the winner-take-all rules are set up, it's going to get easier and easier for Trump to roll up big delegate totals from here on out," said Nevin.

'Trump would get us into war'

Ohio Republican voters like Frank Beierle found Trump's style too harsh for the presidency. The 69-year-old retired mechanic cast his ballot for Texas Senator Ted Cruz instead.

"I'm afraid Trump would get us into war," said Beierle, who lives in the village of St. Bernard. "He comes out too heavy. We need a president that's a little more discreet."

Beierle's wife Sheri voted in Ohio's Democratic primary, which was also held on Tuesday. Distressed by Trump's racially charged rhetoric, she cast her ballot for Hillary Clinton.

"We have two mixed grandchildren, half black grandchildren," Sheri said. "I want their lives to be better than mine and my children's lives."

Clinton increases lead over Sanders

Jarrell Booker, a 23-year-old musician, also expressed concern about race relations in the US. He voted for Hillary Clinton's socialist opponent, Vermont Senator Bernie Sanders.

"I feel like he's for the black people," Booker said. "What's going on with police brutality and all this, he's defending us, standing up as the leader."

Clinton, however, defeated Sanders in both Ohio and Florida. Despite losing in two key states, the Vermont senator will likely remain in the race, powered by passionate grassroots' support.

"It's easy to see a scenario where he stays in the race because he has such a fervent following," said Nevin, a former speechwriter for failed Democratic presidential candidate Martin O'Malley.

"They're funding his campaign, they want to hear his message," Nevin continued. "He's less dependent on winning states than any other candidate in either party in the sense of having a campaign."

Contested Republican convention looms

Trump's loss in Ohio tempered his victory in Florida, where he defeated Senator Marco Rubio by double digits. Rubio exited the race after suffering a crushing defeat in his home state.

Unable to shut down Kasich's long-shot campaign and still facing a formidable Cruz, Trump's path to the Republican nomination has become more complicated.

Though the real estate mogul remains the Republican front-runner, his opponents have racked up a growing number of delegates and could band together to block him at the convention in July.

"It may alienate a chunk of the electorate that is clearly supportive of him, but I don't know where they would go otherwise," said Daniel Birdsong, an expert on US politics at the University of Dayton.

Chris Beckham is part of that electorate. The 19-year-old marketing major believes Trump offers the best chance for conservatives to win in the general election.

"He might be the only one who can unite the Republican Party, who can beat Hillary Clinton," Beckham said. "He gets a lot of independents and Democrats to switch over to the Republican Party."

In Ohio's open primary, Democrats were allowed vote in the Republican race and vice versa.

'Ohio votes like America'

Democratic crossover to the Republican side, however, may have hurt Trump. Although he is a Sanders supporter, Roger Fecher nearly asked for a Republican ballot on Tuesday.

"I thought about voting in that primary just to vote against [Trump]," Fecher said. "But with Sanders still having a chance, we thought it was important to vote for him."

Trump's loss in the Ohio primary doesn't bode well for his chances in the general election.

"Ohio votes like America votes," Nevin said. "It's demographically diverse, it's economically diverse. If you can win here in a primary, its speaks well of your ability to win elsewhere."