Donald Trump: Marco Rubio, Ted Cruz and John Kasich admit they would back him for Presidency

All three of Mr Trump's rivals have launched scathing attacks but would have no choice but to support him as the Republican nominee

They've attacked him for weeks and weeks and said his presidency would be a disaster, but now Donald Trump’s rivals have admitted they will back the billionaire if he wins the Republican nomination.

The mud-slinging continued at Thursday night’s GOP debate, where Ted Cruz claimed the property mogul was “part of the corruption in Washington” and Marco Rubio accused him of manipulating the American public by “telling people whatever you think you need to get them to do what you want them to do“.

But when the last question of the night asked the four remaining Republican candidates whether they would support Mr Trump in the likely event of him winning the nomination, they pulled together for an awkward show of unity.

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“.

”I'll support the Republican nominee,“ said Mr Rubio, who has repeatedly described his rival as a “con man”.

John Kasich, the Governor of Ohio, 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 before saying he would back whoever the Republican nominee is.

It came after a series of senior Republicans attacked Mr Trump, including the party's two most recent presidential nominees, Mitt Romney and John McCain.

Mr Romney, who lost to Barack Obama in 2012, 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.

Mr Rubio said the businessman had ”basically mocked everyone“ and criticising his businesses for making clothes in China and Mexico, rather than the US.

Mr Cruz, from Texas, claimed that 40 years Mr Trump had been ”part of the corruption in Washington“ that people were angry about, citing previous campaign contributions to leading Democrats, including Mrs Clinton when she was a senator.

Protesters are escorted out of a rally for Republican presidential candidate, Donald Trump at Radford University in Radford, Va., Monday, Feb. 29, 2016. (AP)

"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,” Mr Trump said.

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 came a step closer to winning the Republican nomination on Super Tuesday, when he won primaries in seven states, compared to Mr Cruz’s three and one for Mr Rubio.

For the Democrats, Mrs Clinton has a strong lead against Mr Sanders but much could change as states continue to vote in primaries that will continue until June.

Additional reporting by AP