Donald Trump has come under fire again at the latest Republican debate, after a day in which the GOP’s veteran politicians urged voters to desert him.

The front-runner in the Republican race was on the defensive in Detroit as Marco Rubio and Ted Cruz piled in.

Donald Trump admitted he had changed his stance on issues but said flexibility was a strength.

Senior Republicans say Donald Trump is a liability who would lose the election.

The debate hosted by Fox News began with Donald Trump being asked about an attack earlier in the day by Mitt Romney, the 2012 nominee, who accused the businessman of bullying, greed and misogyny.

Donald Trump dismissed Mitt Romney as a “failed candidate”, but he immediately found himself on the defensive from Marco Rubio.

The Florida senator said he was “not going to turn over the conservative movement to someone who thinks the nuclear triad is a rock band from the 1980s”.

In one of the most bizarre moments, Donald Trump defended the size of his hands and then quipped about another part of his anatomy.

There were plenty of personal insults from Donald Trump, who labeled the Florida senator “little Rubio” and the senator from Texas as “liar Ted”.

Donald Trump was forced to explain a civil lawsuit involving the collapse of Trump University.

He said he would win the case but Marco Rubio said he was trying to “con people into giving him their vote, just like he conned people into giving him their money”.

Donald Trump was also challenged by the Fox News panel for changing his stance on Syrian refugees, the war in Afghanistan and President George W. Bush.

He replied: “I have a very strong core. But I’ve never seen a successful person who wasn’t flexible, who didn’t have a certain degree of flexibility.”

Hours earlier, Mitt Romney led growing calls by leading Republicans against a Donald Trump nomination.

Calling him a “phony” and a “fraud”, the former Republican presidential candidate said Donald Trump’s policies – like the deportation of undocumented migrants and banning Muslims from entering the US – would make the world less safe.

Others like Paul Ryan, John McCain and a host of national security committee members have also attacked Donald Trump since he cemented his front-runner status earlier in the week on Super Tuesday.

Republicans in four states – Kansas, Kentucky, Louisiana and Maine – go to the polls on March 5 and Donald Trump is hoping to get a step closer to earning his party’s nomination.

Donald Trump, a billionaire with no experience of political office, has won 10 of the 15 states that have voted so far, with his promise to “make America great again”.

His supporters value his perceived authenticity and business acumen, and say he is the strong leader the country needs.

With the effective departure of Ben Carson this week, the field of Republican candidates – once 17-strong – has now been narrowed to four.

The debate, sponsored by Fox News, was the first time Donald Trump had faced his rivals since winning seven states on Super Tuesday.

It also brought him face to face with presenter Megyn Kelly, whom he dismissed as a “bimbo” after they clashed in the first primary debate.

This time Donald Trump was all smiles and he complimented her looks when he took her first question.

In the Democratic race, Hillary Clinton has 10 states, five more than rival Bernie Sanders.

Hillary Clinton and Bernie Sanders will take to the debate stage in Flint, Michigan, on March 6.