UNITED STATES (AFP) - Mr Donald Trump backed out on Friday (March 4) from appearing at a prominent conservative conference, after plumbing new depths of vulgarity in the latest US presidential debate to the despair of the Republican party.

Mr Trump said he was scrapping a scheduled appearance on Saturday at the Conservative Political Action Conference so he could attend rallies in Kansas, which holds its Republican caucus that day, and then Florida, which votes March 15.

Organisers of the event near Washington said they were "very disappointed" at the front runner's "last-minute" cancellation.

"His choice sends a clear message to conservatives," CPAC said on Twitter.

Earlier that day, Mr Trump had backtracked from statements endorsing the torture of terror suspects and the killing of their families, saying he would not order the US military to break international laws if elected president.

In a statement to the Wall Street Journal, the Republican White House front runner said he would "use every legal power that I have to stop these terrorist enemies".

"I do, however, understand that the United States is bound by laws and treaties and I will not order our military or other officials to violate those laws and will seek their advice on such matters. I will not order a military officer to disobey the law."

Mr Trump's new position stands in contrast to remarks he made at a Republican debate less than 24 hours earlier, when he doubled down on previous pledges that, if elected, he would do "a hell of a lot worse" than waterboarding and said he had "no problem" with the targeting of terror suspects' families.

Republicans face an ever starker choice after Mr Trump's rhetoric stooped shockingly below-the-belt at Thursday's party debate, on whether to embrace his divisive candidacy or derail his march to the nomination.

Within the opening minutes, Mr Trump was referring to the size of his penis, a level of obscenity even by the low standard set in previous showdowns that is raising serious questions in his camp about his electability.

Many conservatives including Mr Ted Cruz were swept into Congress in 2010 and 2012 on a grassroots Tea Party movement, and its co-founder Jenny Beth Martin implored CPAC attendees on Friday to choose Mr Cruz over the front runner.

Mr Trump makes a "seductive pitch" with his call to make America great again, she said, but "Donald Trump loves himself first, last, and everywhere in between".

She blasted him for his flip-flop on immigration and his involvement in a series of real estate courses that accusers say bilked several students out of thousands of dollars.

"Scamming people out of their hard-earned money? Definitely not Tea Party," she said. Despite the escalated attacks against Mr Trump, each one of his debate opponents - Senator Cruz, Senator Marco Rubio and Ohio Governor John Kasich - admitted they would support the party's eventual nominee.

But Friday's criticism of Mr Trump by conservatives appalled by the debate still amounted to a stunning rebuke.

"The spectacle made me ill," wrote conservative Matthew Continetti in the Washington Free Beacon.

"On screen I watched decades of work by conservative institutions, activists and elected officials being lit aflame not only by the New York demagogue but by his enablers who waited until the last possible moment to try and stop him."

Conservative messaging guru Frank Luntz, who conducted a focus group during the debate, sounded apoplectic about the vulgarity.

"This has to stop," he tweeted, adding that "22 out of my 25 focus group members said tonight's #GOPDebate will hurt Republicans in the general election".

Senior figures, including 2012 Republican nominee Mitt Romney, have expressed concern that a Trump nomination could help propel Democrat Hillary Clinton to the presidency.

"Donald Trump is a phony, a fraud," Mr Romney said on Thursday as he urged voters to support any one of his challengers for the party nomination.

Mr Trump wasted little time in striking back, calling Mr Romney a "choke artist" and assailing him for "begging" for an endorsement, only to lose to President Barack Obama four years ago.

Thursday's debate marked the nadir of a campaign season already notable for its extreme, unruly tone. With the bitter rivals gathered at a pivotal moment in the campaign, the debate attacks turned deeply personal.

Mr Rubio insisted it was Mr Trump who opened the flood gates with his criticism of women, immigrants, Muslims and the disabled.

"If there has been any candidate who deserved to be attacked that way it's Donald Trump for the way he has treated people in the campaign," Mr Rubio said.

Mr Matt Schlapp, chairman of the American Conservative Union that hosts CPAC, had a suggestion for all the Republican candidates: "Tone it down."

Whether or not Mr Trump should carry the torch is now the crux of the GOP race, which he has dominated since jumping in eight months ago.

Mr Cruz insisted "the stakes are too high" to let Mr Trump be the nominee, arguing that Mrs Clinton will eat him alive ahead of the November election. "Nominating Donald would be a disaster," Mr Cruz said.

Mr Cruz has won four of the 11 states so far. Mr Rubio, seen by many as the most logical mainstream alternative to Mr Trump, has won just one, Minnesota. Trump has won the rest.