TAMPA: 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 laboured to stop his outsider candidacy needed to recognise 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 U.S. 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 cancelled 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."

'TIME TO UNITE'

Her voice hoarse, Clinton said it was "time for us to unite as a country."

White House spokesman Josh Earnest criticized Trump's Republican rivals for declaring they would back him if he wins the party nomination for the November election.

"At some point, somebody in the Republican Party's going to have to step up and show some leadership," Earnest said.

Trump has emerged from the early contests with a clear lead in the delegates needed to capture the nomination at the party's July convention. US Senator Ted Cruz of Texas is second, with Rubio and Kasich trailing and counting on their home states to keep them in the race.

Trump's contentious campaign has been marked by harsh rhetoric against illegal Mexican immigrants and Muslims. There have been repeated clashes in recent days between his supporters and protesters, raising questions about whether the violence will hurt Trump in the primary race.

Rubio made a last-minute appeal to supporters at a rally at a college in West Palm Beach, Florida, speaking to a larger-than-normal crowd of 1,600, many of them students at the school.

He sounded alarmed at the prospect of Trump winning Florida and capturing the nomination.

"Leadership is not inciting people to get angrier,” Rubio said. “That's not leadership. You know what it is? That's called demagoguery."

'PUNK ASS THUGGERY STUFF'

Former Alaska Governor Sarah Palin, who appeared at a Trump event in Tampa on Monday, said protesters were "trying to take away your rights" to gather peacefully.

"What we don’t have time for is for all that petty, punk ass thuggery stuff that has been going on," she said, before she headed home to be with her husband, Todd, who was injured in a snowmobile accident.

Cruz held five rallies in the Chicago suburbs and told voters to support him if they wanted to stop Trump.

He said Trump had donated in the past to two Illinois Democrats - former Governor Rod Blagojevich and Chicago Mayor Rahm Emanuel - and asked voters to "remember who the moneybags is that's funding these politicians."

In the Democratic race, Clinton, a former secretary of state, hopes to pull away from Sanders, a US senator from Vermont, in Tuesday's voting. Polls gave her a big lead in Florida and North Carolina, but showed Sanders gaining ground in Ohio, Illinois and Missouri.

Sanders' win last week in Michigan, where polls indicated he trailed by double-digit margins, showed his ability to pull off a surprise.

He told a town hall sponsored by MSNBC airing on Monday that Trump was "literally inciting violence among his supporters" by offering to pay their legal fees.