Donald Trump unleashed fiery, off-color rhetoric at Thursday's Republican debate, as American conservatives mull embracing his divisive candidacy or finding a way to derail the controversial billionaire's march to the nomination.

With the real estate tycoon apparently on a glide path to becoming the Republican standard-bearer, Trump faced a day of relentless criticism from party leaders, with some panic setting in at the prospect of nominee Trump.

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

With Trump and his three Republican rivals -- Senator Ted Cruz, Senator Marco Rubio, and Ohio Governor John Kasich -- gathered at a pivotal moment in the campaign, the debate attacks turned deeply personal and bizarre, with Trump making a startling if veiled reference to his own genitals.

"He hit my hands. Nobody has ever hit my hands," Trump said, referring to Rubio's recent personal attack in which he stated that the celebrity billionaire has small hands.

"He referred to my hands," Trump went on as the raucous crowd laughed and booed.

"If they are 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 provocative and assaulting tone.

Rubio and Kasich, who is scrambling to avoid irrelevance in the fierce nomination contest, both said they hoped the race could focus on important policy issues and not personal debasement.

"I have never tried to go and get into these kind of scrums that we are seeing here," Kasich stressed.

"People say wherever I go: 'You seem to be the adult on the stage.'"

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, Romney said, "is playing the American public for suckers."

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

Romney still holds sway with certain elements within the party, but even Kasich downplayed the influence that the 2012 nominee would have on 2016.

"Mitt Romney is a great guy, but he doesn't determine my strategy," Kasich said at the debate.

TRUMP ‘DISASTER’

The debate, broadcast on Fox News, took place in the largest city in Michigan, the biggest prize of the four states holding Republican primaries next Tuesday.

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

Cruz insisted "the stakes are too high" to let Trump be the nominee, arguing that Clinton will eat him alive ahead of the November election.

"Is this the debate you want playing out in the general election?" Cruz asked, arguing that Trump's legal problems with his closed university and his financial contributions to Clinton's 2008 presidential campaign would spell huge trouble.

"Nominating Donald would be a disaster," Cruz said.

Trump insisted he was the only one capable of defeating Clinton.

"I have not started on Hillary yet," Trump said. "Believe me, I will start soon."

Several conservatives gathering at an annual convention outside Washington said they were torn.

"I supported Rubio... now I'm going to have to hold my nose and vote for Trump," Ron Fodor, mayor of the small town of Slippery Rock, Pennsylvania, said at the Conservative Political Action Conference (CPAC).

House Republican Steve King backs Cruz, but he said forcing a party implosion just to stop a popular candidate was not the answer.

"We should not change the rules just because you don't like the person that emerges in the leadership," King told AFP.

The anti-Trump coalition is not the issue, argued Senator Ben Sasse, who is desperate to derail the tycoon.

"We have a frontrunner right now who has waged war against almost every core plank of the platform, so it isn't any anti-movement that's causing that problem," he said.

Still, the #NeverTrump movement was making a stand at CPAC, where Brian Hawkins, a 27-year old African-American waving a "Veterans Against Trump" sign, said it was time to elevate an alternate candidate.

"A lot of us are thoroughly offended that these elements of the Republican Party have somehow had the loudest voice or are on the verge of nominating Trump," Hawkins said.