Republican front-runner Donald Trump has warned of riots if he is denied the party's presidential nomination and pulled the plug on a scheduled debate among candidates, raising the temperature even more in a heated White House race.
The outspoken New York businessman scored big wins in primaries in Florida, Illinois and North Carolina on Tuesday, bringing him closer to the 1237 convention delegates he needs to win the nomination.
Trump also claimed victory in Missouri but lost the crucial state of Ohio, and left the door open for those in the party trying to stop him from becoming the Republican nominee for the 8 November election.
Trump might fall short of the majority of delegates required, enabling the party's establishment to put forward another name at the July convention in Cleveland to formally pick its candidate.
In an interview with CNN on Wednesday, Trump said the party could not deny him the nomination should he fail to win enough delegates.
"I don't think you can say that we don't get it automatically. I think you'd have riots. I think you'd have riots. I'm representing many, many millions of people."
While the Republicans were mired deeper in turmoil, Hillary Clinton won victories in at least four states on Tuesday that put her in good shape to defeat Senator Bernie Sanders of Vermont and win the Democratic Party's nomination.
Republican Party leaders are appalled at Trump's incendiary rhetoric and reject policies such as his vow to deport 11 million illegal immigrants, temporarily ban Muslims from the United States and build a wall along the Mexican border.
The party tried to play down his riot comments, only days after Trump supporters and protesters clashed at a rally for the Republican in Chicago that was later scrapped.
"First of all, I assume he is speaking figuratively," Republican National Committee spokesman Sean Spicer told CNN.
Recent outbreaks of violence during protests at Trump rallies have prompted President Barack Obama, a Democrat, and mainstream Republican figures to speak out against the billionaire.
David Farber, a professor of modern American history at Temple University in Philadelphia, said that, though there have been many episodes of violent unrest at American political events, not least the 1968 Democratic convention, Trump's warning of a riot was unprecedented.
The nearest modern comparison would be former Alabama Governor George Wallace, who unsuccessfully ran for president in 1968 as a third-party candidate, Farber said.
"We're in a situation somewhat like the 1960s in the United States, a period of great polarisation," Farber said, "and there just isn't a lot of common ground, and Trump supporters feel a lot of quiet desperation that Trump is helping to fan the flames of."
Trump boasts of his 'very good brain'
In comments likely to raise more concern in the Republican establishment about Trump's lack of experience and temperament, the former reality TV show host said he was for the most part his own foreign affairs adviser.
"I'm speaking with myself, number one, because I have a very good brain," he told MSNBC's "Morning Joe" show. "I know what I'm doing. ... My primary consultant is myself."
Trump's closest national challenger is first-term US Senator Ted Cruz of Texas, who prides himself in being a grassroots conservative often at odds with Republican leaders.
He too warned of severe reactions against an attempt to stage a so-called brokered convention or contested convention to install a Republican candidate supported by party leaders.
"I think that would be an absolute disaster. I think the people would quite rightly revolt," Cruz told CNN.
A brokered convention is a complicated process of sequential votes that opens the way for horse trading.
The Republican establishment's bid to stop Trump may have come too late as the field of candidates has dwindled to only three, with Trump, 69, in command ahead of Cruz, 45, and Ohio Governor John Kasich, 63, who won his state's Republican primary on Tuesday and is the last moderate Republican presidential candidate standing.
Growing in confidence, Trump pulled out of a Republican debate scheduled for Monday in Utah, saying it clashed with a speech he plan to give to a pro-Israel group. Debate hosts Fox News then cancelled the event.
Senator Marco Rubio quit the White House race after defeat in his home state of Florida.
Trump now needs to win about 55 percent of the roughly 1100 delegates still up for grabs in state-by-state nominating contests to guarantee the nomination. It is not an insurmountable challenge.
Republican strategist Ron Bonjean said it might be tough for the party to block Trump at the convention.
"A contested convention would be justified if Trump only had around 35 or 40 percent of the delegates locked up. However, if he is very close to getting the majority of delegates, it would be politically difficult for the establishment to try stop him by backroom wheeling and dealing without risking a serious backlash from voters," said Bonjean. The strategist is not affiliated with any of the candidates.
Party figures are divided about whether to throw their weight behind Trump despite his downsides or to go on trying to halt him. Florida Governor Rick Scott endorsed Trump on Wednesday but another influential Southern governor, South Carolina's Nikki Haley, declared her support for Cruz, the state's Post and Courier newspaper said.
The election season is likely to become more politicised after Obama nominated judge Merrick Garland to the Supreme Court, setting up a showdown with Senate Republicans who have vowed to block any Obama nominee.
On the Democratic side, wins on Tuesday for former Secretary of State Clinton, 68, gave her an almost insurmountable edge over Sanders, 74. His campaign said it was not ruling out asking for a recount in Tuesday's tight Missouri race.
Seeking to become the United States' first woman president, Clinton needs to win only around a third of the Democratic delegates remaining to become her party's nominee.