WASHINGTON: Republican leaders and mainstream voters were watching Friday for signs that Thursday's attacks on front-running presidential candidate Donald Trump, one of the most extraordinary days in party history, had blunted his campaign for the party's nomination.

Trump was sharply criticized by 2012 nominee Mitt Romney and faced debate assaults by Ted Cruz and Marco Rubio as the party elite scrambled to block the trash-talking New York billionaire businessman's path to the November ballot.

The chaos reflects the Republican leadership's conviction that Trump cannot defeat likely Democratic nominee Hillary Clinton, the former secretary of state and first lady.

Still, Texas Senator Cruz, Florida Senator Rubio and Ohio Governor John Kasich all declared during Thursday night's debate that they would support Trump if he won the primary election battle. Trump, in turn, said he would support whoever wins, though he seemed to find it inconceivable that it might not be him.

Romney's speech, an unheard-of attack by a former party candidate for the nation's highest office, signaled a state of desperation inside the party establishment.

"His is not the temperament of a stable, thoughtful leader," Romney declared. He called Trump "a phony" and "a con man" who is "playing the American public for suckers," a man whose "imagination must not be married to real power."

Sen. John McCain, also a former Republican nominee, endorsed Romney's attack.

Trump lashed back in the debate, calling Romney "a failed candidate" who lost to President Barack Obama four years ago because he was such a poor candidate.

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

He said it was fine that Rubio had negotiated with other lawmakers on immigration policy. He said he had changed his mind to support allowing in more highly skilled workers from abroad, adding, "I'm changing. I'm changing. We need highly skilled people in this country."

And he was matter-of-fact about providing campaign contributions to leading Democrats, including 10 checks to Hillary Clinton in her 2008 primary contest with Obama. She is 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.

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

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.