A supporter for US Republican presidential candidate Donald Trump holds up a sign during a campaign event in Hickory, North Carolina March 14, 2016. [Photo/Agencies]

TAMPA, Fla. - US Republican presidential front-runner Donald Trump hunted for votes in three states on Monday before a crucial round of nominating contests, and dismissed outbreaks of violence at his campaign events as "a little disruption."

Trump, who made appearances in North Carolina, Florida and Ohio ahead of Tuesday's primaries, said the establishment Republicans who have labored to stop his outsider candidacy needed to recognize his strength and rally to his cause.

The billionaire businessman trained his fire on rival Republican candidate, Ohio Governor John Kasich, at an evening rally in Youngstown, Ohio. Kasich is running neck and neck with Trump in the state.

"Your coal industry is dead. Your steel industry is dead," Trump told Ohio voters. "Your governor is totally overrated. He hasn't done a thing."

Kasich is looking to win his home state and thwart Trump's progress toward securing the 1,237 delegates a candidate needs to win the nomination.

"I'm going to win Ohio, and it's going to be a whole new ballgame," Kasich said on Fox News on Monday. "We're not going to lose tomorrow."

Ohio, Florida, Illinois, North Carolina and Missouri hold nominating contests on Tuesday for the Nov. 8 election to succeed Democratic President Barack Obama.

Trump could seize control of the Republican race with a sweep and possibly knock out two of his rivals, Kasich and US Senator Marco Rubio of Florida. Opinion polls show him leading in all five states except Ohio.

"If we win Ohio and we win Florida, then everybody agrees it's pretty much over," Trump told an afternoon rally in Tampa, Florida. "The Republican Party has to come together."

In Hickory, North Carolina, Trump rejected suggestions his combative campaign tone was to blame for recent clashes at his rallies, including one last week where a protester was punched and a Chicago rally that was canceled after fights between Trump supporters and opponents.

He said the level of violence was inflated by the media and "basically" no one had been hurt at his rallies, although "maybe somebody got hit once."

"It's a little disruption, but there's no violence," Trump said, describing his campaign as "a movement and a lovefest."

The Republican Party's anti-Trump forces, including a Super PAC formed to oppose him, kept up their assault on the brash New York real estate mogul. The Super PAC released an ad on Monday featuring demeaning quotes by Trump about women, and urged people to oppose him "if you believe America deserves better."

Mitt Romney, the 2012 Republican presidential candidate who has delivered a furious attack on Trump, campaigned for Kasich in North Canton, Ohio, on Monday but refrained from criticizing Trump by name or explicitly endorsing Kasich.

"He has the kind of record you want in Washington," Romney said of Kasich.

Democratic presidential front-runner Hillary Clinton also kept an eye on Trump, saying at a campaign event in Chicago that "I don't think the stakes have ever been higher, or the rhetoric on the other side ever been lower."