US Democratic hopeful Bernie Sanders has hit back after Republican front-runner Donald Trump said he had sent protesters to disrupt his rallies.
The row came after Mr Trump was forced to call off a rally in Chicago on Friday after violent clashes.
Mr Sanders called Mr Trump a “pathological liar”.
He and his rival Hillary Clinton also accused Mr Trump of encouraging supporters to beat up protesters and then offering to pay their legal costs.
On Tuesday Republicans and Democrats vote in primaries in states including Florida and Ohio.
Republican candidate Marco Rubio, currently in third place, has said a win in his home state Florida is vital to prevent the Republican nomination being “hijacked” by Mr Trump.
Reacting to Mr Trump’s allegations, Mr Sanders also told the billionaire to tell his supporters that violence was not acceptable in the political process.
Meanwhile Mrs Clinton accused the Republican of conducting a cynical campaign of hate and fear.
Hundreds of protesters gathered at the venue at the University of Illinois at Chicago on Friday. Inside the auditorium, fighting broke out between supporters and protesters, who waved flags and chanted.
A statement from Mr Trump’s campaign said the candidate decided to postpone the event after meeting with police.
However, a Chicago Police Department spokesman said the force had not recommended that Mr Trump postpone the rally.
Violent clashes continued outside the venue, with helicopter footage showing chaotic scenes as police attempted to control the large crowds.
A day later, on Saturday, a man tried to rush Mr Trump as he spoke on stage in Ohio but was denied by Secret Service agents.
Mr Trump has denied using hate speech or playing any part in fostering division.
“I represent a large group of people that have a lot of anger,” he said. “There is tremendous anger out there on both sides.”
But his rivals for the Republican nomination have blamed him for encouraging his supporters to use violence against any dissenters.
A number of Trump rallies have been interrupted by protesters before.
These latest clashes come just a day after a Trump supporter was charged with assault after multiple videos showed him punching a protester at a campaign rally in North Carolina.
The billionaire later said that the supporter’s actions were “appropriate”.