(AP) -- Relentless in their attacks, Marco Rubio and Ted Cruz leveled withering criticism at Donald Trump's "flexible" policy positions and personal ethics in a Republican presidential debate Thursday that also featured a crude sexual reference from Trump.

The two senators, who earlier had devoted considerable debate time to throwing sharp elbows at one another, pressed Trump aggressively on his conservative credentials, his business practices and changing policies.

But on a day when the Republican establishment was in chaos over the prospect of Trump landing the GOP nomination, Cruz, Rubio and Ohio Gov. John Kasich all said they would support Trump if he won primary election battle. And Trump, in turn, said he would support whoever wins -- though he seemed to find it inconceivable that it might not be him.

Pressed on policy matters, Trump, in short order, signaled a willingness to deal on any number of issues.

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

He said he had changed his own mind to support admitting more highly skilled workers from overseas, adding 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.

The bad blood among the candidates flowed freely.

Rubio justified his attacks on Trump by saying the billionaire businessman had "basically mocked everybody" over the past year. Trump countered with a feint, saying he'd called Rubio a "lightweight" in the past but "he's really not that much of a lightweight."

Trump then noted that Rubio had mocked his hands as small, widely viewed as an insult about Trump's sexual prowess. Holding his hands up to the audience, Trump 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.

On policy, 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."

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

In one moment of levity, Texas Sen. Cruz poked fun at Trump for interrupting, telling the businessman, "Breathe, breathe, breathe."

Rubio chimed in, "When they're done with the yoga, can I answer a question?"

In one particularly sharp exchange, Rubio renewed his criticisms of Trump University, which charged students $1,495 each for seminars that would teach them the billionaire's secrets to making it big in real estate. A lawsuit filed by the New York attorney general claims the classes fell so short of promises that it amounted to fraud.

"He's trying to do to the American voters what he did to the people that signed up for this course," Rubio declared. "He's trying to con people into giving him their vote just as he conned those people."

Cruz, tag-teaming on the issue, saying, "If we nominate Donald, we're going to spend the fall and the summer with the Republican nominee facing a fraud trial,"

Trump was dismissive, saying: "It's a minor civil case. Give me a break."

And he turned the tables, saying Rubio is "the real con artist." He said the first-term senator "scammed the people of Florida" by skipping a high number of votes while running for president.

In another exchange, Rubio faulted Trump's businesses for manufacturing clothing in China and Mexico rather than the U.S. 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.

Cruz, too, took the fight to Trump, 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.

Thursday's debate was 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 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."

With Ben Carson's exit from the race this week, the field of Republican candidates has now been narrowed to four, but any number of predictions that GOP voters would unite behind one anti-Trump candidate have come and gone without a change in the overall dynamic.

Trump, with 10 state victories, 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.

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Benac reported from Washington.

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