Donald Trump unleashed fiery, off-color rhetoric at Thursday's Republican debate after enduring a day of intense criticism by party leaders, as conservatives agonize over embracing his divisive candidacy or derailing his march to the nomination.

But despite the escalated attacks against Trump by his rivals on the debate stage, each one of them - Senators Ted Cruz and Marco Rubio and Ohio Governor John Kasich - acknowledged they would support the party's eventual nominee.

With the real estate tycoon apparently on a glide path to becoming the Republican standard-bearer, some panic has set in at the prospect of Trump winning the nomination.

Other operatives and voters said it is time, for better or worse, to rally around the man leading the pack.

With the bitter rivals gathered at a pivotal moment in the campaign, the debate attacks turned deeply personal, even vulgar. Trump made a startling if veiled reference to his genitals as he hit back against Rubio for mocking the size of Trump's hands.

"Nobody has ever hit my hands," Trump said as the raucous crowd laughed and booed.

"He referred to my hands. If they're small, something else must be small. I guarantee you there is no problem."

Rubio excused his own attacks by insisting it was Trump who opened the flood gates. "Donald Trump has basically mocked everybody with personal attacks," Rubio said.

Trump's remarks, likely unprecedented in a US presidential debate, appeared as the nadir of a campaign season already notable for its unruly, coarse tone.

With Trump thrown on the defensive, he lashed out multiple times, at one point hurling insults and talking over his rivals. "You've defrauded the people of Florida, little Marco," he said.

Florida will vote on March 15 and is Rubio's firewall, even though Trump leads there in polls. If he cannot win his home state, Rubio will find no path to the nomination.

Kasich, who is scrambling to avoid irrelevance in the fierce nomination contest, said he hoped the race could focus on important issues and not the "scrums" and personal debasement on display Thursday.

With time running out to stop Trump, Mitt Romney - who ran unsuccessfully against Barack Obama in 2012 - on Thursday offered up some of the harshest criticism yet, lambasting Trump as unfit to be president.

Romney said a Trump nomination would enable a Democratic victory for the party's presumptive nominee Hillary Clinton.

"Donald Trump is a phony, a fraud," Romney said in a speech in Utah, as he urged voters to support one of the remaining candidates.

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

AFP - AP