CINCINNATI - White House hopefuls hit the ground in a last-minute push for votes Monday ahead of a crucial new election test, with violence marring Donald Trump's race for the Republican nomination.

Dubbed "Super Tuesday 2" by US media, the latest major date in the run-up to November's presidential election will see Democratic and Republican primary contests in the states of Florida, Ohio, Illinois, Missouri and North Carolina.

The billionaire Trump is looking to build an insurmountable lead, but Republican rivals Texas Senator Ted Cruz, Florida Senator Marco Rubio and Ohio Governor John Kasich are more determined than ever to bar his path.

Trump has jetted across the delegate-rich states in recent days, staring down criticism over clashes at a rally on Friday in Chicago - which many saw as a natural consequence of the violent tone of his campaign rhetoric.

The Republican frontrunner's invective has targeted immigrants, Muslims, Hispanics and other minorities, journalists and the disabled - often to raucous approval from thousands of chanting partisans.

But, as with each new controversy swirling around him, Trump seemed unscathed by the uproar with polls suggesting he remained on a glide path toward his party's nomination heading into Tuesday's make-or-break round of voting.

Trump, 69, has rejected out of hand any suggestion that his rhetorical excesses have created a climate of violence, blaming supporters of Bernie Sanders for sowing trouble - and threatening to respond in kind by sending supporters to picket the Democratic candidate's rallies.

The 74-year-old senator from Vermont, who has pointedly refrained from personal attacks in the campaign, gave a furious retort at a CNN Democratic town hall event on Sunday, stating bluntly: "Donald Trump is a pathological liar."

"We have never, our campaign does not believe in and never will encourage anybody to disrupt anything," Sanders added. "It is clear that Donald Trump is running a very cynical campaign pitting groups of Americans against one another. He is trafficking in hate and fear," Democratic candidate Hillary Clinton said.

Trump's Republican rivals, eager to bring him down a notch but also seeming shaken by the weekend violence, have pounced on the frontrunner. Rubio, who is trailing in third place and like Kasich faces a do-or-die test in Tuesday's vote in his home state, called Trump's language "dangerous."

"If we reach a point in this country where we can't have a debate about politics without it getting to levels of violence and anger," Rubio told CNN, "we're going to lose our republic."

Kasich has accused Trump of creating a "toxic" atmosphere. Friday's troubles in Chicago saw ardent Trump supporters and opponents come to blows, after dozens of campaign stops where he has encouraged the crowd to verbally and physically mistreat protesters.

Trump appeared Sunday to condone one particularly striking act of violence, when a demonstrator was sucker-punched as he was led by police from a rally last week in North Carolina.

Trump said his staff would look into paying the belligerent supporter's legal bills.

Tensions were palpable at Trump's campaign stops, with Secret Service and police ringing the candidate, after Friday's chaotic scenes and a brief security scare caused the next morning when a protester burst on stage at an event in Ohio.

Trump went on to claim the protester was linked to the Islamic State group - an assertion that he refused to disown, despite it becoming apparent it was based on a crude video hoax.

"What do I know about it?" he told NBC. "All I know is what's on the Internet."

The man in question, a 22-year-old anti-racism activist named Thomas DiMassimo, told CNN Sunday he had been seeking to wrench the microphone from Trump - and had no desire to cause harm.

"I was thinking that Donald Trump is a bully, and he is nothing more than that," DiMassimo said. "He's truly just a coward. And he's opportunistic and he's willing to destroy this country for power for himself."

In Boca Raton, Florida, on Sunday Trump held a generally peacefully rally at an outdoor amphitheater, attended by several thousand people. "I'm a better person than the people I'm running against, that I can tell you," Trump told his cheering supporters. "And I'm a better person than Hillary. Hillary is not good."

Earlier in the day in Cincinnati, Ohio, hundreds lined up to see Trump despite the drizzle - while demonstrators massed chanting "Build bridges, not walls" and "No Trump, no KKK, no fascist USA."

A protester holding a Sanders sign interrupted the candidate before being escorted out, to loud cheers from the crowd. "It's fine," Trump said. "In certain ways, it makes it more exciting."

Trump supporter Adam Ward, a 34-year-old Iraq war veteran, said he believed the protesters were actually helping the candidate. "It enrages people that don't agree with Bernie Sanders," he said. "I probably wasn't going to come to this until I saw Chicago being shut down." Trump heads to rallies Monday in Ohio, Florida and North Carolina.