CHICAGO — Donald Trump's campaign abruptly canceled his rally here Friday night over security concerns as protesters clashed with his supporters inside an arena where he was to speak.
Protesters have interrupted virtually every Trump rally, and Trump's appearance here — on the campus of a large, diverse public university just west of downtown Chicago — had drawn anger long before it began. Trump's security has tried to identify and exclude potential demonstrators before they enter his events, but hundreds of protesters were able to get into the arena and fill several sections at the University of Illinois at Chicago Pavilion, and some of them engaged in skirmishes with Trump's fans.
"The event is over," an announcer said, again and again, trying to disperse the tense crowd.
The protesters burst into cheers and chants. Trump supporters, some of whom had waited in the stands here for hours, appeared stunned by what had happened, and some shouted at members of the media and the vocal group of protesters.
In a statement, Trump's campaign said he had "just arrived in Chicago and after meeting with law enforcement has determined that for the safety of all of the tens of thousands of people that have gathered in and around the arena, tonight's rally will be postponed to another date."
"Thank you very much for your attendance and please go in peace."
The canceled rally came as Trump, the front-runner for the Republican presidential nomination, sought to move past the primary fight, saying that there had been enough debates and that the party needed to come together behind him. But as he cast his eye forward, he raised fresh questions about his porous understanding of intricate policy matters that all presidents face.
At a morning news conference at his Mar-a-Lago complex in Florida, Trump collected the endorsement of Dr. Ben Carson, his former rival, and said that Republicans must begin to unite to reclaim the White House in the fall.
"We've had enough debates," Trump said. "How many times do you have to give the same answer to the same question?"
But those looking to halt Trump's march toward the nomination had other ideas. A top aide to Sen. Marco Rubio of Florida, who is fighting for his political life in the primary there Tuesday, urged voters to back the candidate best positioned to stop Trump, even if it meant that Rubio supporters in certain states should vote for a rival candidate to prevent a Trump victory there.
The Republican contests Tuesday are seen as potentially climactic: A loss by Rubio in Florida or Gov. John Kasich of Ohio in his home state would likely force either man to exit the race. A Trump victory in one of those states would dramatically accelerate his march toward the nomination, and a victory in both could all but end the primary race, even as Sen. Ted Cruz of Texas battles on.
At Trump's news conference making the case for the party to unite behind him, he said he would win states in a general election that would otherwise be "unthinkable" for the Republican Party. He allowed that there might be "two Donald Trumps," suggesting there's a private, more palatable version that doesn't align with the showier public version, before he reversed the statement a few minutes later. He defended violence against some of the protesters at his increasingly tense rallies, suggesting that they have often instigated the problems.
"Let me tell you — we've had some violent people as protesters," Trump said.
The New York businessman made the comments the morning after a debate in which he sought to appear presidential and was pressed at length on details of his foreign policy views, exposing surprising and, to some, alarming positions on issues such as how to handle the quagmire in the Middle East. Military experts raised eyebrows at his statement that he would consider deploying 20,000 to 30,000 U.S. ground troops in Syria and Iraq.
Those experts said it did not reflect any current thinking among military experts, even by the most hawkish analysts.
"I know of no active-duty or recently retired military officers who fought in Iraq or Syria who would say this was a good idea," said Mark P. Hertling, a retired three-star general who commanded 30,000 U.S. troops in northern Iraq. "I'm not sure where any of those numbers are coming from. None of them are based on what we call troop-to-task analysis."
Hertling said the United States might have to deploy additional resources to Iraq when the Iraqi military prepared to retake the country's second-largest city, Mosul, from the Islamic State. But under the plans being prepared by the Pentagon, that would consist of advisers to help collect and analyze intelligence or to pilot drones, not ground troops.
"We can't put ground forces in to fight this," he said, "or we're going to get bogged down."
Trump has repeatedly failed to make good on promises to release a list of foreign policy advisers, and his spokeswoman, Hope Hicks, did not respond to an email about how he arrived at the troops estimate.
Meanwhile, a top aide to Rubio said that the best way to stop Trump on Tuesday, when voters in a slew of Midwest states as well as Florida and North Carolina will head to the polls, was for the senator's supporters in Ohio to vote for Kasich in the primary.
"I'm just stating the obvious," said Alex Conant, a spokesman for Rubio. "If you are a Republican primary voter in Ohio and you want to defeat Donald Trump, your best chance in Ohio is John Kasich, because John Kasich is the sitting governor. He's very close to Donald Trump in some of the polls there."
The remarks dovetail with a strategy proposed by Mitt Romney, the party's 2012 nominee, who urged Republicans opposing Trump to coalesce around the leading non-Trump candidate in coming nominating contests to deny the nomination to the New York businessman.
But Rubio's rivals were not on board with the idea. Speaking in Orlando, Florida, on Friday, Cruz dismissed the Rubio campaign's gambit, indulging in a laugh when asked about it.
"It's the Washington establishment's last gasp: 'Let's divide things up. Let's play games,'" Cruz said in a taped interview with Fox News. "It's real, real simple. How do you beat Donald Trump? You beat him."
Kasich also was not thrilled with the idea, saying his supporters should vote for him if he's on the ballot.
"What kind of a deal would it be if I told my people, 'Don't vote for me'?" he told reporters in Moraine, Ohio, on Friday evening.
Highlighting the stakes, Trump began airing new ads in Florida and Ohio, one of which attacks Rubio as corrupt and another of which flays Kasich for his ties to Wall Street because of his work for Lehman Bros.
Meanwhile, in Miami Beach, dozens of leading donors to the Republican National Committee gathered Friday morning at the Ritz-Carlton in South Beach to hear about on the party's progress on data collection and voter outreach.
At one session, two committee officials, including the party's chief counsel, John Phillippe, also delivered a briefing on the delegate math in the nominating fight and the mechanics of a contested convention. The briefing drew so many questions that Reince Priebus, the party chairman, reconvened the session again after lunch.
Asked by one donor how close the nomination was to being decided, Republican National Committee officials indicated that any candidate, including Trump, would need more than half the remaining delegates to win the nomination outright at the July convention.
Some donors concluded from the presentation that Trump, for all his dominance so far, could still be beaten.
Trump's campaign also announced a team of four advisers, three of whom worked for Carson's campaign, to track and maintain commitments from delegates leading up to the Republican National Convention. Trump's previous lack of such staff had been of deep concern to some of his allies, given the potential ability of rival campaigns to pick off delegates.
Clashes with protesters have been a growing issue at Trump rallies, but the crowd in Chicago was especially diverse, and small skirmishes had been building among the growing crowd inside for hours before the event was set to start.
The university draws a significant share of local students from Chicago's neighborhoods, as well as international students. Scores of faculty members at the school, the University of Illinois at Chicago, had pleaded with administrators over allowing the rally in a letter, which read, in part: "We also request that the university publicly distance itself from the event and make a statement that the Trump rally is an anathema to the mission of UIC, as the university for Chicago."
Chicago, a city that is split almost in thirds white, black and Latino, has been in turmoil over questions of race and policing for months. The release in November of a video of the fatal shooting of Laquan McDonald, a black teenager, by a white Chicago police officer had set off weeks of protest.
Chicago police, along with university officers, federal authorities and others, were out in force Friday. Anthony Guglielmi, a spokesman for the Chicago police, promised a "very visible" presence of officers.
A spokesman for the Chicago police said Friday evening that there had been no arrests, and at that point, no reports of injury. The spokesman said that Chicago authorities were not consulted and had no role in the choice to cancel the event.