CHICAGO/PALM BEACH, Fla. (Reuters) - U.S. presidential candidate Donald Trump canceled a campaign rally Friday night amid security concerns just hours after the Republican front-runner earned the endorsement of a former rival who said the billionaire’s pugnacious campaign style is belied by a more thoughtful, private side.

U.S. Republican presidential candidate Donald Trump speaks at the Peabody Opera House in St. Louis, Missouri, March 11, 2016. REUTERS/Aaron P. Bernstein

“There are two different Donald Trumps: there’s the one you see on the stage and there’s the one who’s very cerebral, sits there and considers things very carefully,” Ben Carson said Friday as he became the second former Republican candidate to back Trump in the White House race.

The soft-spoken retired neurosurgeon, who dropped out of the race last week, said the American people would be “comforted” when they discover Trump’s gentler side.

The thousands of protesters who showed up for Trump’s rally Friday evening at the University of Illinois at Chicago - along with thousands of supporters - showed little indication they had noticed anything but the candidate’s combative campaign style.

The university arena turned into a chaotic scene as the two warring sides amped up their positions. A half hour after the rally was slated to begin, a Trump campaign staffer announced it was being postponed for safety reasons, unleashing competing chants of “We dumped Trump!” and “We want Trump!” throughout the packed venue.

“We made a great decision not to have the rally,” Trump told CNN after meeting with law enforcement and making the call.

“I am not a person that wants to see violence,” he added.

Trump blamed protesters for creating disturbances at his campaign events and said it is a “love fest” among his supporters.

Friday’s event in Chicago stood out because the huge number of protesters virtually matched the number of Trump supporters, as opposed to other Trump campaign events where protesters have been a very small, albeit vocal, minority.

Earlier in the day, speaking at a public event in St. Louis, Missouri, Trump was interrupted repeatedly by protesters who were led out of the event by police and security, an increasingly common occurrence at his raucous rallies.

“He’s all mouth, get him out,” Trump shouted as one of the protesters was led out. “Go back to mommy,” he said as another protester was led away.

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The latest endorsement for Trump followed a Republican debate in Miami on Thursday night at which Trump and the remaining three candidates in the Republican race struck a markedly more civil tone.

Carson shot to the top of the Republican pack last year but faltered in the early nominating contests. His endorsement is unlikely to dramatically shift the Republican race, but it gives Trump a boost as the Republican establishment cranks up attacks, and comes just days before crucial nominating contests in the battle to be the party’s presidential candidate for the Nov. 8 election.

The Republican primaries to be held on Tuesday in five states will be critical for Trump to cement his lead, and to determine whether U.S. Senator Marco Rubio of Florida and Ohio Governor John Kasich, whose home states are among those holding contests on Tuesday, will be able to continue with their increasingly long-shot candidacies. Trump’s nearest rival in the race is U.S. Senator Ted Cruz of Texas.

Carson’s comments on Friday aimed to soften Trump’s public image after a campaign marked by his demeaning personal attacks on opponents, harsh comments about Mexican immigrants and calls to temporarily bar all Muslims from entering the country.

“I’M A THINKER”

Trump’s controversial campaign has led many Republican establishment figures to call for an all-out effort to prevent him from winning the nomination - an effort that Carson said would fracture the Republican Party and ensure a Democratic win in November.

Asked about Carson’s comments, Trump said he did not want to “overanalyze” himself but there was only “one Donald Trump.”

“Certainly you have all of this, and you have somebody else that sits, and reads and thinks. And I’m a thinker,” said Trump, 69. “Perhaps people don’t think of me in that way because you don’t see me in that form.”

Trump also raised the possibility that he will not attend the next Republican debate, scheduled for later this month in Salt Lake City. “We’ve had enough debates, in my opinion,” he said.

In St. Louis, Trump’s speech was interrupted more than a half-dozen times by protesters. Scuffles between Trump supporters and protesters have become more frequent, and a protester was punched in North Carolina on Wednesday by a Trump supporter who has been charged with assault.

“The officers are being very gentle,” Trump advised, telling the crowd later, “It adds to the flavor, makes it more exciting, isn’t this better than listening to a long boring speech?”

Carson’s endorsement of Trump followed that of another former candidate, New Jersey Governor Chris Christie, who announced his backing last month.

Next Tuesday’s voting will be a pivotal point as candidates chase the 1,237 delegates from primary contests and caucuses needed to win the Republican nomination. Trump has 459 delegates, followed by Cruz at 360, Rubio at 152 and Kasich at 54, according to the Associated Press.

The primaries in Florida and Ohio on Tuesday have the potential to be game-changers because both states award Republican delegates on a winner-take-all basis, meaning that he winner of the popular vote in each state will be awarded the state’s entire slate of delegates. Many states award delegates proportionate to the popular vote.

For his part, Rubio said in a round of television interviews on Friday he was still in position to win Florida next week. Voters in his home state who do not want Trump as the Republican nominee should support him, he said.

“If they don’t want Donald Trump to be our nominee, then voting for John Kasich or Ted Cruz in Florida is a vote for Donald Trump,” Rubio said on ABC’s “Good Morning America.”

Rubio said later that his supporters in Ohio should vote for Kasich next Tuesday if that looked like the best anti-Trump tactic.

“Clearly John Kasich has a better chance of winning Ohio than I do, and if a voter in Ohio concludes that voting for John Kasich gives us the best chance to stop Donald Trump there, I anticipate” that is what they will do, Rubio told reporters at an event in West Palm Beach, Florida.