DETROIT: US Republican presidential front-runner Donald Trump defended changing positions on issues from Syrian refugees to immigration on Thursday at a debate where he faced wave after wave of attacks as the party's establishment sought to unite behind a last-ditch anti-Trump effort.

Tempers escalated quickly during a two-hour Fox News Channel debate with tensions mounting over the New York billionaire's ascendancy and his drive to be the presumptive nominee should he win nominating contests in Florida and Ohio on March 15.

Trump, presented with videotaped evidence from the Fox moderators that he had shifted positions on the Iraq war, immigration and whether to admit refugees from the Syrian civil war, shrugged.

"You have to show a degree of flexibility," he said.

His answer provided fodder for more attacks from rivals Marco Rubio, a US senator from Florida, and Ted Cruz, a US senator from Texas, who are anxious to stop the rise of Trump.

"I hope we don’t see yoga on this stage," Cruz said. Replied Rubio: "Well, he's very flexible so you never know."

It remained to be seen whether the debate would prove to be damaging to Trump. The runaway front-runner to date has been immune from criticism that other politicians normally face, for instance, over flip-flopping on issues.

Despite the ill will between them, both Rubio and Cruz said reluctantly they would support Trump should he win the nomination. Ohio Governor John Kasich, 63, agreed while noting, "Sometimes he makes it a little bit hard."

Asked if he would support the Republican nominee if it's not him, Trump said, "Yes I will."

The debate was the four remaining candidates' first face-to-face gathering since Super Tuesday nominating contests this week gave extra momentum to Trump but did not knock out his rivals.

Mainstream figures in the party have been seeking a strategy to halt Trump's march to the nomination for the Nov. 8 election to succeed Democratic President Barack Obama.

At center stage, Trump, 69, defended himself from criticism earlier in the day from 2012 Republican nominee Mitt Romney and faced further questions about his business record. Trump called Romney a failed candidate.

IMMIGRATION, FOREIGN WORKERS

Rubio, 44, and Cruz, 45, questioned Trump's immigration policy and his use of foreign workers at his exclusive Mar-a-Lago resort in Palm Beach, Florida.

Cruz demanded Trump release an audiotape of an off-the-record session he had with New York Times editorial writers on Jan. 5.

Cruz and others have suggested that in the session Trump might have been more flexible on immigration than in public statements insisting he would build a wall between the United States and Mexico and deport 11 million illegal immigrants.

Trump refused to release the tape but said he would be flexible, for instance, on the height of the wall. He also abruptly changed his position on foreign workers, saying more of them who are highly skilled should be allowed to remain in the United States.

Rubio pressed Trump on the foreign workers he has imported to work at his Palm Beach resort, jobs he said could go to Americans. Trump said the workers were for a short November-to-March season.

"People don’t want a short-term job," Trump said. "So we bring people in and we send people out."

'LITTLE MARCO,' 'BIG DONALD'

Rubio asked Trump why he does not bring his clothing-making operations to the United States from China and Mexico if he is so interested in bringing jobs home, a central tenet of his unconventional campaign.

"This little guy has lied so much about my record," Trump said in response to Rubio, adding that he had begun bringing some clothing operations home from overseas.

But Rubio persisted: "The answer is he’s not going to do it ... The reason he makes it in China and Mexico is because he can make more money on it."

"Don't worry about it, little Marco, I will," Trump said dismissively.

"Well, let's hear it, big Donald," Rubio responded.

Fox News moderator Megyn Kelly, who famously clashed with Trump at the first Republican debate last August, generated a fresh exchange in pressing Trump to explain his involvement with Trump University, a now-defunct online education company that has faced lawsuits from people who feel they paid out money for Trump U and got nothing in return.

"Give me a break," said Trump. "Let's see what happens in court. This is a civil case. It's very easy to have been settled."

Rubio accused Trump of fleecing everyday Americans for personal gain.

"He's trying to do to the American voter what he did to the people who signed up for this course," Rubio said.

Trump called Rubio a "con artist" for missing a lot of Senate votes.

The debate went down a negative path early on when Trump responded to Rubio's contention last month that Trump had "small hands."

"Look at these hands," Trump said, flashing his two hands to the crowd. He dismissed any suggestion he might be small elsewhere, Trump said: "I guarantee you there is no problem."