Presidential contender Donald Trump has come under attack at a Republican debate, hours after being hammered by some of the party's veteran politicians.
The front-runner in the race to be the party's nominee took to the stage in Detroit with rivals Marco Rubio and Ted Cruz keen to make their mark.
Mr Rubio said Mr Trump deserved to be attacked because he had mocked others.
Many Republicans fear Mr Trump would lose November's presidential election.
Mitt Romney, the 2012 nominee, earlier lambasted the businessman.
"Think of Donald Trump's personal qualities, the bullying, the greed, the showing off, the misogyny, the absurd third grade theatrics," said Mr Romney.
Calling him a "phony" and a "fraud", the former standard-bearer of the party said Mr Trump's policies - like the deportation of undocumented migrants and banning Muslims from entering the US - would make the world less safe.
Other senior Republicans like Paul Ryan, John McCain and a host of national security committee members have also attacked the New Yorker since he cemented his front-runner status on Super Tuesday.
Mr 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".
Texas Senator Mr Cruz, with four states, and Florida Senator Mr Rubio, with one, joined Mr Trump on stage at 21:00 local time (02:00 GMT) after days of launching blistering attacks on the businessman.
With the effective departure of Ben Carson this week, the field of Republican candidates - once 17-strong - has now been narrowed to four.
The fourth Republican still competing, John Kasich, is hopeful he can win in his home state of Ohio, where voters go to the polls on 15 March.
On Thursday morning, Mr Trump predicted he would be assailed from all sides in the debate.
"I can't act overly presidential because I'm going to have people attacking from every side," he told NBC.
"A very good man, Ben Carson, is not there any more, so now we're going to have more time for the fighting."
He was also dismissive of Mr Romney's criticisms, describing him as a failed candidate who once begged for Mr Trump's endorsement.
The debate, sponsored by Fox News, is the first time Mr Trump will face his rivals since winning seven states on Super Tuesday.
It will bring him face to face with presenter Megyn Kelly, whom he dismissed as a "bimbo" after they clashed in the first primary debate.
The incident prompted him to boycott a subsequent debate hosted by Fox.
In the Democratic race, Hillary Clinton has 10 states, five more than rival Bernie Sanders.
The two will take to the debate stage in Flint, Michigan, on Sunday.