Romney, who has blasted Trump for days, delivered a sharp condemnation of Trump earlier Thursday -- a deeply unusual move for a former presidential nominee.
"Here's what I know: Donald Trump is a phony, a fraud," Romney said in a speech in Salt Lake City. "His promises are as worthless as a degree from Trump University. He's playing members of the American public for suckers. He gets a free ride to the White House, and all we get is a lousy hat."
He hit back during the debate, calling Romney an "embarrassment" to the GOP.
"He was a failed candidate, he should have beaten President Obama very easy. He failed miserably and it was an embarrassment to everybody," Trump said. "I guess obviously he wants to be relevant, he wants to be back in the game."
During a rally in Maine earlier Thursday, Trump blasted Romney for "begging" for his endorsement.
"I don't know what happened to him," Trump said. "You can see how loyal he is. He was begging for my endorsement. I could have said, 'Mitt, drop to your knees.' He would have dropped to his knees."
With Trump emerging as the undisputed GOP front-runner, many party leaders and insiders insist he's not a true conservative. They also worry that Trump's inflammatory rhetoric on immigration could permanently unravel the GOP's efforts over the years to reach out to Latinos and other minorities.
But even as Romney and others try to lead the effort to stop Trump, the belated push may only reinforce the real estate mogul's appeal as his supporters have already shown their deep loathing for establishment figures.
Amid the GOP establishment's revolt against Trump, all three of Trump's rivals pledged on the debate stage to support the party's eventual nominee even if that ends up being Trump.
Trump, who has repeatedly threatened the possibility of launching a third-party bid should he feel that the party is treating him unfairly, also said he would back whoever the party crowns.
"The answer is, yes, I will."
Donald Trump has come under attack from his rivals at a Republican debate, after a day in which the party'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.
In a testy debate, Mr Trump admitted he had changed his stance on issues but said flexibility was a strength.
Senior Republicans say Mr Trump is a liability who would lose the election.
The debate began with Mr 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. Mr Trump dismissed Mr Romney as a "failed candidate".
Ohio Governor John Kasich was the fourth Republican contender to take part in Thursday's debate.
Leading the attack on Mr Trump, Florida Senator Marco Rubio 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, Mr Trump defended the size of his hands and then quipped about another part of his anatomy.
There were plenty of personal insults from Mr Trump, who labelled the Florida senator "little Rubio" and the senator from Texas as "liar Ted".
Since Super Tuesday, Donald Trump has faced relentless, withering criticism from his own party.
He was challenged on his various policy changes, on his failed businesses, on his support for Democrats.
He told the audience he reserved the right to be "flexible" and change his mind on issues if he felt like it. He was shown tapes of all the times he'd done just that.
The mud-slinging continued and at one point Ted Cruz urged Mr Trump to "breathe, breathe, breathe". But it is the Republican Party who appear to be gasping. They are trying to stop the front-runner, fearing he will destroy the party. The problem is Trump supporters have so far proved to be loyal no matter what is flung at their man.
The New York businessman was forced to explain a civil lawsuit involving the collapse of Trump University.
He said he would win the case but Mr Rubio said he was trying to "con people into giving him their vote, just like he conned people into giving him their money".
Mr 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."
The debate, sponsored by Fox News, brought Mr Trump face to face with presenter Megyn Kelly, whom he dismissed as a "bimbo" after they clashed in the first primary debate.
This time he was all smiles and he complimented her looks when he took her first question.
