Donald Trump is embracing what he calls flexibility on issues like torture and illegal immigration, abandoning at least for now the tough rhetoric that has fuelled his rise to Republican front-runner status. His critics, as well as fellow Republican candidates, remained skeptical Friday.

In the days after his dominant Super Tuesday primary performance, Trump is using more moderate tones and downplaying his call for a temporary ban on Muslims entering the United States.

The billionaire reality TV star has shot to the top of the primary contest with denigrating remarks about Hispanics immigrants and Muslims. But in Thursday's chaotic debate, he signalled his willingness to compromise on immigration, among other key issues.

He said he was retreating from the anti-visa position advocated in a position paper posted on his website, one of the few specific policies his team has released during the campaign.

"I'm changing. I'm changing. We need highly skilled people in this country, and if we can't do it, we'll get them in," he said in the debate.

More broadly, Trump insisted that compromise would be part of any immigration reform.

That didn't sit well with challenger Ted Cruz, the Texas senator.

"'Flexible' is Washington code word that he's going to stick it to the people," said Cruz, who holds second place in the number of Republican delegates collected so far on the way to the party's nominating convention this summer.

Trump faces s growing list of high-profile Republicans who denounce him as dangerous, not a true Republican and lacking the experience to lead the world's most powerful nation.

Mitt Romney, the Republican presidential nominee four years ago, declared Friday that he would not vote for Trump if he were the nominee. He told NBC that he would "do everything within the normal political bounds to make sure we don't nominate Donald Trump."

Still, some members of Romney's vast donor network said they were ready should he reconsider. Chicago Republican donor Bill Kunkler said he had recently spoken to Romney's 2012 finance chairman, Spencer Zwick, and told him he would support Romney again.

"Mitt is the guy who will put the party before himself," Kunkler said.

On Thursday, dozens of conservative national security experts wrote an open letter pledging to oppose Trump's candidacy in part because of his "embrace of the expansive use of torture."

Trump responded to such concerns in a statement Friday, saying that he understands that the US is "bound by laws and treaties" and he will not order US military officials to violate or disobey those laws if elected president. It appeared to be a retreat from declarations that he would bring back the use of waterboarding and that he would target the wives and children of suspected extremists.

If Trump's immigration shift did not bother conservatives, his decision to cancel a scheduled weekend appearance at the nation's largest annual gathering of conservative activists did.

"Very disappointed @realDonaldTrump has decided at the last minute to drop out of #CPAC -- his choice sends a clear message to conservatives," the American Conservative Union, which hosts the Conservative Political Action Conference, said in a Twitter post.

Trump's campaign said the cancelled because of newly scheduled rallies in Kansas and Florida.

Trump shrugged off Friday's wave of criticism by unleashing a verbal assault on his Republican rivals at a Detroit-area rally. He repeatedly called Florida Sen. Marco Rubio "Little Marco" and Cruz "Lying Ted."

Despite the verbal abuse, Cruz, Rubio and Ohio Gov. John Kasich all declared during Thursday'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.

So far, Trump has 10 state victories and 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.