The raucous debate capped a day that saw the Republican's most recent presidential nominees, Mitt Romney and John McCain, lambasting Trump, the current front-runner, calling him unfit for office and a danger for the nation, in an extraordinary show of intra-party chaos.

The growing feud marked a near-unprecedented scenario pitting the Republican Party's most prominent leaders, past and present, against each other as Democrats begin to unite around Hillary Clinton.

Underlying the clash is a bleak reality for panicking Republican officials: Beyond harsh words, there is little they see to stop Trump's march toward the presidential nomination.

The chaos was reflected in the back-and-forth at the Republicans' first post-Super Tuesday debate, where Trump repeatedly clashed with the remaining candidates.

Rubio justified his attacks on Trump by saying the billionaire businessman had "basically mocked everybody" over the past year.

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

It was a jaw-dropping moment in a campaign that's been full of surprises from the beginning.

For all of the criticism and ill will, Cruz, Rubio and Kasich all said they would support Trump if he is the Republican nominee. He, too, said he would support whoever wins.

There were moments of policy debate, too, as Rubio and Cruz pressed Trump aggressively on his conservative credentials, his business practices and shifting policy positions.

Trump, in short order, demonstrated his willingness to deal and be flexible when it suits his needs.

He said it was fine that Rubio had negotiated with other lawmakers on immigration policy.

He acknowledged changing his own mind to support admitting more highly skilled workers from overseas, saying matter-of-factly, "I'm changing. I'm changing. We need highly skilled people in this country."

And he also was matter of fact about providing campaign contributions to leading Democrats, including 10 checks to Hillary Clinton, reviled by many conservatives.

Trump said it was simply business.

"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 said.

When Rubio faulted Trump's businesses for manufacturing clothing in China and Mexico rather than the U.S., Trump retorted, "This little guy has lied so much about my record."

Asked when he would start making more clothes in the U.S., Trump said that would happen when currency valuations weren't biased against manufacturing garments in America.

When moderator Megyn Kelly told Trump his shifts caused some people to question his core, Trump insisted: "I have a very strong core. 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."

John Kasich sought to turn Trump's statement on the value of "flexibility" into a character question. When the Ohio governor meets with voters, he said, "you know what they really want to know? If somebody tells them something, can they believe it?

Cruz, too, took the fight to Trump, saying that while it's easy to print campaign slogans on baseball caps, as Trump does, the question is whether Trump understands what made America great in the first place.

He labeled Trump part of the problem, not the solution, accusing him of being "someone who has used government power for private gain."

"For 40 years, Donald has been part of the corruption in Washington" that people are angry about, Cruz said, citing Trump's campaign contributions to leading Democrats, including then-Sen. Clinton.

Trump piled more insults, too, on the party's 2012 presidential nominee, Mitt Romney, who earlier Thursday made a rare public appearance to denounce Trump as "a phony" who is "playing the American public for suckers."

Trump dismissed Romney as "a failed candidate" and an "embarrassment."

"Obviously, he wants to be relevant," Trump said dismissively. "He wants to be back in the game."

With Ben Carson's exit from the race this week, the field of Republican candidates has now been narrowed to four, including Texas Sen. Cruz and Ohio Gov. Kasich.

But any number of predictions that Republican voters would unite behind one anti-Trump candidate have come and gone without a change in the overall dynamic.

Trump, with 10 state victories, continues to dominate the conversation and the delegate count.

Thursday's debate, sponsored by Fox News, was the first time Trump faced his rivals since scooping up seven victories on Super Tuesday.

It was also the first time he faced questioning from Kelly since the two clashed in the first primary debate. That's when Kelly's tough questioning about Trump's treatment of women blew up into a running argument between Fox and the candidate. Trump, who dismissed Kelly as a "lightweight" and a "bimbo," ended up boycotting a subsequent Fox debate, claiming the network was unfair.

Trump signaled he was ready for a truce. When Kelly posed her first question to him, Trump told her "you're looking well. You're looking well."

Trump has continued to pile up delegates during the long, and so far unsuccessful, effort to topple him.

He leads the field with 329 delegates. Cruz has 231, Rubio 110 and Kasich 25. It takes 1,237 delegates to win the Republican nomination for president.

Associated Press writers David Eggert in Detroit and Nancy Benac contributed to this report.

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