Republican presidential candidates Marco Rubio and John Kasich have suggested they may not support Donald Trump if he becomes the Republican nominee, as violence at the front-runner's rallies deepened the party's chaotic chasm.
Tensions ran high at Mr Trump's latest rally, when US Secret Service agents briefly formed a protective ring around the candidate, then left the stage and allowed him to continue speaking at an airport hangar near Dayton, Ohio.
It was not immediately clear why the agents rushed the stage to surround Mr Trump, who appeared to jolt after hearing something coming from the audience behind him.
A defiant Mr Trump has denied that he has encouraged violence at his events. But the scenes from his aborted rally in Chicago on Friday night appeared to be a final straw for some rivals who had pledged, despite deep concerns about his qualifications, to support the billionaire businessman if he were to become the nominee.
Mr Rubio, a Florida senator, said it was "getting harder every day" to keep his word. Mr Kasich, the Ohio governor, said the "toxic environment" Mr Trump is creating "makes it very, extremely difficult" to support him.
"To see Americans slugging themselves at a political rally deeply disturbed me," Mr Kasich said while campaigning in Cincinnati. "We're better than that."
The extraordinary shift by the two came just a few days before Tuesday's elections in five states, including Florida, Ohio and Illinois.
Mr Trump insisted he had done nothing to exacerbate tensions, despite having previously encouraged his supporters to aggressively - and sometimes physically - stop protesters from interrupting his raucous rallies.
"I don't take responsibility. Nobody's been hurt at our rallies," Mr Trump told CNN late on Friday, one of several interviews he did as cable networks broadcast footage of the skirmishes both inside and outside the Chicago arena where he had planned to speak.
In Dayton, the audience chanted Trump's name as the Secret Service agents rushed the stage. Mr Trump did not explain what had happened, but said: "Thank you for the warning. I was ready for 'em, but it's much better if the cops do it, don't we agree?"
The brash billionaire's unexpected political success has caused turmoil in the Republican Party.
Most leaders expected his populist appeal would fade as nominating contests began and largely avoided criticising even his most extreme comments out of fear of alienating his supporters.
But after 24 primary contests, Mr Trump has only grown stronger and leads his rivals in the all-important delegate count. The delegates will select the party's nominee at the Republican national convention in July in Cleveland.
Republican leaders are grasping for a last-ditch idea to stop Mr Trump from claiming the nomination, from talking about a contested convention to discussing whether to rally around a yet-to-be-determined third-party candidate. All are long shots at best and would likely have the effect of ripping the Republican Party apart in irreparable ways.
Mr Rubio and Mr Kasich must win their home state contests on Tuesday in order to stay in the race.
Texas senator Ted Cruz, closest to Mr Trump in the delegate count, has urged both to drop out so he can take on the front-runner in a head-to-head contest.
Mr Cruz said late on Friday that Mr Trump has created "an environment that encourages this sort of nasty discourse."
"When the candidate urges supporters to engage in physical violence, to punch people in the face, the predictable consequence of that is that is escalates," he said.
The chaos in Chicago was sparked in part by Mr Trump's decision to cancel his rally after skirmishes broke out in the crowd that, unlike past Trump events, was packed with protesters.
Some isolated confrontations took place afterwards. Police reported arresting five people. Many anti-Trump attendees had rushed on to the floor of the University of Illinois at Chicago Pavilion, jumping up and down with their arms up in the air.