WASHINGTON: Donald Trump's gladiator-style politics may have finally caught up with him, as the Republican's rivals accuse him of turning the White House race into a violent circus and analysts warn of more trouble ahead.

Can the genie be put back in the bottle now that mounting tensions pitting Trump's angry, frustrated crowds against equally fired-up protesters have tipped over into violence?

There is increasing doubt on the question, says Steffen Schmidt of Iowa State University who grimly predicts “tensions and violence at political events will increase sharply” on the long road to the November election.

In a campaign already marked by foul language and below-the-belt attacks in the Republican camp, the weekend of March 11-13 marked a shocking new low.

First, Trump was forced to cancel a rally on Friday as ardent Trump supporters and opponents came to blows in chaotic scenes in Chicago.

Then, early Saturday, a man tried to rush on stage in Ohio before Secret Service quickly stopped him and surrounded Trump.

That same evening, police used pepper spray on anti-Trump protesters in Kansas City, Missouri, saying they had been infiltrated with “known anarchists, outside agitators".

The weekend violence came after dozens of campaign stops where Trump has encouraged the crowd to verbally and physically mistreat protesters.

Rowdy standoffs between pro- and anti-Trump protesters — often with a racial edge to them — appear to have become the norm at the billionaire real estate mogul's rallies.

After accusing Democratic candidate Bernie Sanders of infiltrating his rallies with disruptive protesters, Trump vowed to send his own militants to the Vermont senator's gatherings.

Sanders, speaking at a CNN townhall Sunday, had a simple response: “Donald Trump is a pathological liar."

Read: Trump accuses Democratic candidate of inciting protests at his rallies

Bloody noses, broken bones

Does this mean the clashes, insults and beefed-up security presence are now part and parcel of this White House race? Some fear it openly.

“Trump has lit a fire. Can it be contained?” asked The Washington Post.

Not likely, says Schmidt.

“Trump supporters will now go to Sanders events and 'disrupt' them. Then Sanders supporters will hit back and there will be many bloody noses and broken bones at these and also other Trump events,” he predicted.

“Politics is not a polite game,” he said. “Politics is a contact sport."

Trump has created many foes in what just nine months ago seemed no more than a quixotic endeavour, fanning the flames of hatred and emboldening his most ardent supporters along the way.

By unleashing diatribes against Washington, insults against his rivals — from “little” Marco Rubio to “liar” Ted Cruz — stigmatising Muslims and Hispanics, Trump stands accused of fanning hatred and spurring his partisans to violence.

The Republican frontrunner has openly spoken of wanting to “punch... in the face” or “knock the crap” out of opponents, and said he would consider paying the legal fees for a supporter who sucker-punched someone at a Trump rally.

Take a look: Trump supporter punches protester at rally

Yet the bombastic billionaire refuses to admit any responsibility for violence, insisting his rallies are friendly, peaceful affairs even as he continuously pushes the boundaries of acceptability with his verbal provocations.

“His rhetoric is inciting the thuggery spilling over at his rallies,” a Post editorial read.

Trump has justified violent outbursts by some of his supporters in the name of rightful “anger".

In such a climate, it is little wonder his rallies have become a magnet for anti-racism activists, defenders of immigrants and minorities as well as libertarian and anarchist groups.

Genie out of bottle

“Trump has attracted these protester-types — they are not Bernie Sanders's... but fringe-types and fringe-personalities and hard-core activists who want to engage,” Duke University associate professor Pope “Mac” McCorkle told AFP.

“It might have some short-term gains in driving even more of his committed people to the polls, but as a long-term it's a loser.

“It's a bit like the genie out of the bottle: I think these demonstrators are going to keep on appearing at his rallies and I don't know how you put it back into the bottle."

It was already common for one or a handful of protesters to disrupt Trump's speeches, only to be thrown out of the room.

But in Chicago on Friday, there were thousands.

Trump called them professional “thugs” and agitators behind a "planned attack".

The incidents earned Trump a sharp rebuke from President Barack Obama, who urged the candidates vying to replace him to reject “insults and schoolyard taunts” and “violence against other Americans".

Democratic frontrunner Hillary Clinton bluntly accused Trump of "political arson".

Trump's three remaining rivals for the nomination, seizing a chance to bring the frontrunner down a notch, have also been unanimous in condemning the rally chaos.

“I think it is going to be a rocky road out there for a while,” McCorkle said.

If Trump and Clinton win their party nominations, Schmidt predicted, “the campaign after this summer is going to be one of the nastiest presidential campaigns since Thomas Jefferson,” in the early 19th century.