Donald Trump's rivals pledge support if he wins, after Mitt Romney attack

Mr Romney, the 2012 nominee, made a rare public appearance to denounce Mr Trump as "a phoney" who is "playing the American public for suckers".

It highlighted the unprecedented way the party's most prominent leaders are feuding, as panicking Republican officials realise there is little they can do stop Mr Trump's march towards the presidential nomination.

The debate in Detroit was the first since the tycoon's Super Tuesday success, and saw more personal attacks than previous ones.

But senators Ted Cruz and Marco Rubio, and Ohio governor John Kasich, all said they will support Mr Trump if he wins the Republican nomination.

Mr Rubio said that he is committed to his party and therefore would support the nominee.

Mr Cruz said he would prefer Mr Trump to either of the Democratic contenders, Hillary Clinton or Bernie Sanders, describing the latter as a "socialist".

Mr Kasich was the most hesitant, but ultimately said that while "sometimes (Trump) makes it a little bit hard," he would support him if he is the nominee.

Mr Trump was also asked if he would support someone other than himself, and hesitated briefly, but said that he would back whoever the Republican nominee is.

However there was less unity during the raucous debate, with Mr Rubio saying the billionaire businessman had "basically mocked everyone" and criticising his businesses for making clothes in China and Mexico, rather than the US.

Mr Cruz, from Texas, said Mr Trump was part of the problem, not the solution, as he had used government power for private gain.

He said for 40 years the tycoon had been "part of the corruption in Washington" that people were angry about, citing Mr Trump's campaign contributions to leading Democrats, including Hillary Clinton when she was a senator.

Mr Trump said: "I've supported Democrats and I've supported Republicans, and as a businessman I owed that to my company, to my family, to my workers, to everybody to get along."

He responded to Mr Rubio's comments by retorting: "This little guy has lied so much about my record."

Pressed on when he would start making more clothes in the US, he said that would happen when currency valuations were not biased against manufacturing garments in America.

Mr Trump then noted that Mr Rubio had mocked his hands as small, widely viewed as an insult about the tycoon's sexual prowess, and, holding his hands up to the audience, he declared: "I guarantee you, there's no problem."

Earlier the tycoon had hit back at Mr Romney, dismissing him as a "failed candidate" and an "embarrassment".

Mr Trump said: "Obviously, he wants to be relevant, he wants to be back in the game."