United States Republican presidential frontrunner Donald Trump has come under fire from rivals Marco Rubio and Ted Cruz at a debate as establishment Republicans tried to muster some unity behind a last-ditch anti-Trump effort.

The debate began on Thursday night in Detroit as a free-for-all fracas with tension mounting over Trump's ascendancy and his drive to become the presumptive nominee should he win nominating contests in Florida and Ohio on March 15.

At centre stage, Trump defended himself from criticism earlier in the day from 2012 Republican nominee Mitt Romney and faced further questions about his record as a businessman at the debate.

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Rubio, a US senator from Florida, pressed Trump on why he does not bring his clothing-making operations to the US from China and Mexico if he is so interested in bring jobs home, a central tenet of his unconventional campaign.

"This little guy has lied so much about my record," Trump responded, 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.

The debate quickly went down a negative path 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. To the suggestion that he might be small elsewhere, Trump said: "I guarantee you there is no problem."

Support for Clinton

Cruz, a US senator from Texas, suggested Trump would be the wrong candidate to send into battle against Democratic frontrunner Hillary Clinton because he has supported her financially in the past.

"Actually it was for business," Trump replied. "Let me tell you something, Ted, the last person that Hillary Clinton wants to face is Donald Trump."

Trump was joined on stage at the Fox Theatre by his three remaining rivals, Rubio, Cruz and Ohio Governor John Kasich.

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It is a far smaller field than the 17 Republican candidates that began the race for the 2016 presidential nomination, but one that is still splintered between the incendiary New York businessman and three experienced politicians.

The debate was the 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 are seeking a strategy to halt Trump's march to the nomination for the November 8 election to succeed Democratic President Barack Obama.

Some party leaders and donors are critical of Trump's positions on trade and immigration, including his calls to build a wall between the US and Mexico, deport 11 million illegal immigrants and temporarily bar Muslims from entering the country.

At the debate, Trump, 69, was questioned for the first time since last year by Fox News anchor Megyn Kelly, who angered him with her questioning at the Republicans' first debate on August 6, prompting him to cancel participation in a debate in Iowa in January, a move that appeared to cost him some votes.

Despite the tension, towards the end of the debate, all candidates - including Trump - vowed to support whomever the presidential nominee would be.

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The debate took place hours after 2012 Republican nominee Romney gave a blistering speech about the dangers of choosing Trump. He called Trump a phony and a fraud who has failed in many business ventures despite his touted success.

The Detroit debate was one more opportunity for Rubio and Cruz to try to slow Trump's momentum. They are the last two anti-Trump candidates standing in what has been a bruising nomination battle. Kasich has largely steered clear of the anti-Trump effort and tried to remain above the fray.

Rubio went on the attack against Trump at the last debate on February 25 and has tried to establish himself as the main Trump alternative by labeling him as a "con artist" who has escaped serious vetting by a news media fixated on his star power and brash rhetoric.