YOUNGSTOWN, Ohio (AFP) - The six remaining White House hopefuls made a frantic push for votes Monday on the eve of make-or-break nominating contests, with Donald Trump's Republican rivals desperate to bar his path after a weekend of violence on the campaign trail.
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, Illinois, Missouri, North Carolina and Ohio.
Violent clashes and protests at Trump rallies over the weekend dominated US headlines, with rivals in both parties accusing the billionaire real estate mogul of creating a toxic campaign environment.
But the latest polls showed the Republican frontrunner in a position to win every state except Ohio, where John Kasich, the state's governor, holds a slender lead in some surveys.
A Kasich victory in Ohio may be the last chance to derail Trump's march to the Republican nomination in July, especially as the party moves to a winner-takes-all format in terms of the delegates accorded for each primary win.
Florida, Illinois and Ohio are the biggest prizes on Tuesday because each state offers a high number of delegates.
In a sign of the stakes involved, Trump cancelled a rally Monday in Florida, where he has a 20-point lead over Florida Senator Marco Rubio, to campaign in Ohio.
Also campaigning in Ohio was Senator Bernie Sanders, who drew about 2,000 people at a rally in Youngstown, Ohio in his bid to stop Democratic frontrunner Hillary Clinton, who is poised to extend her lead if polls prove accurate.
Clinton is easily ahead in Florida, but Ohio appeared to be a tougher contest, where the former secretary of state only held a five-point lead, according to the latest Quinnipiac University poll released Monday.
The clashes that erupted at a Trump rally in Chicago marked an escalation in the tensions that have trailed the Republican candidate.
His invective against immigrants, Muslims, Hispanics and other minorities have become a staple of his campaign, drawing roars of approval from his supporters but also increasingly aggressive protests.
On Saturday, a protester rushed the stage as Trump was winding up a speech near Dayton, Ohio.
"I was thinking that Donald Trump is a bully, and he is nothing more than that," Thomas DiMassimo, a 22-year old college student who was charged with disorderly conduct and inciting panic, told CNN.
Mike Luggery, a 26-year-old veteran at a Trump rally in Boca Raton, Florida, said Sunday the emotions Trump arouses were "an outlet for frustration" among voters who have lost out to globalisation.
Trump, 69, has rejected out of hand any suggestion that his rhetorical excesses have created a climate of violence, blaming Sanders supporters for sowing trouble - and threatening to respond in kind by sending supporters to picket the Democratic candidate's rallies.
The 74-year-old Vermont senator, who has pointedly refrained from personal attacks in the campaign, responded bluntly at a CNN Democratic town hall event: "Donald Trump is a pathological liar." Clinton accused the Republican frontrunner of engaging in "political arson." "He is trafficking in hate and fear," she said.
Trump's Republican rivals also seemed shaken by the ugly turn in a campaign that has for months been immersed in mudslinging and name-calling.
Rubio, who 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." 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 agents and police ringing the candidate.
Trump claimed the protester who rushed the stage in Ohio 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."