Herman Cain's economic plan was attacked as a tax increase waiting to happen Tuesday night in a fiery debate that also saw animosities flare anew between Gov. Rick Perry and former Massachusetts Gov. Mitt Romney.
Perry and Romney swapped criticism in unusually personal and biting terms.
"You have a problem with allowing someone to finish speaking," Romney declared as the two men interrupted one another repeatedly in a disagreement over immigration, one of several vigorous clashes they had.
The fifth debate in six weeks ranged over familiar contentious territory -- from immigration and health care to the economy and energy, often in antagonistic terms. (See also: Fact-Checking Tuesday's GOP Debate)
The candidates engaged each other more directly and sometimes more heatedly than in previous debates.
Romney's Mormon faith also came up, and Perry said he disagreed with a pastor and political supporter who described the religion as a cult. "I can't apologize any more than that," Perry said.
"That's fine," responded Romney.
Candidates assail Cain's 9-9-9 plan
Cain's unlikely rise from asterisk in the polls to contender was clearly on the minds of his rivals.
Rep. Michele Bachmann of Minnesota led the verbal assault moments after the debate began, saying his call for a 9 percent federal sales tax would only be the beginning, with the rate rising later.
Former Sen. Rick Santorum of Pennsylvania wasn't nearly as gentle, citing one analysis that found that taxes would go up for 84 percent of the nation's households if Cain's proposal went into effect. "We're talking about major increases in taxes," he said, adding that a single person and a couple with children with the same income would pay the same tax under Cain's proposal.
Undeterred, Cain insisted the charges were untrue. He said he was being criticized because lobbyists, accountants and others "want to continue to be able to manipulate the American people with a 10-million- word mess," the current tax code.
The former pizza company CEO's proposal is for a 9 percent personal income tax, a 9 percent corporate tax and a 9 percent national sales tax.
"Herman, I love you, brother, but let me tell you something, you don't need to have a big analysis to figure this thing out," Perry said to Cain. "Go to New Hampshire where they don't have a sales tax and you're fixing to give them one," he said, referring to the state that will hold the first primary early next year.
Tough words on immigration
It was Perry who instigated the confrontation over immigration, saying that Romney had no credentials on the issue because he had once hired an illegal worker, the "height of hypocrisy."
Romney denied the charge, saying he had hired a company to mow his lawn and did not know that it had an illegal immigrant on its payroll.
The two men talked over one another, and at one point, Romney placed his on Perry's shoulder.
"It's been a tough couple of debates for Rick. And I understand that, so you're going to get testy," he said.
As Perry continued to speak, Romney stopped him: "You have a problem with allowing someone to finish speaking, and I suggest that if you want to become president of the United States, you've got to let both people speak," he said.
On a more substantive level, Perry said he opposed repealing the portion of the 14th Amendment to the Constitution that says anyone born in the United States is automatically a citizen.
Bachmann, Santorum and Rep. Ron Paul of Texas all sidestepped the question.
Cain apologized for earlier remarks about building an electric fence on the Mexico border that could kill people trying to cross illegally.
Cain found himself on the defensive on two others issues during the two-hour debate.
He apologized for earlier remarks about building an electric fence on the Mexico border that could kill people trying to cross illegally.
And he said he wouldn't be willing to negotiate with terrorists, even though he suggested he might be in an interview earlier in the day.
Foreign policy took a secondary role in the debate, and the new strain of Republican isolationism quickly surfaced.
Paul said U.S. troops should be withdrawn from Korea -- where they have been stationed for more than 50 years -- and foreign aid to Israel cut.
Perry said it was "time to have a very serious discussion about defunding the United Nations."
Related Coverage: Fact-Checking Tuesday's GOP Debate
|Tracking Rick Perry:
For the latest on Rick Perry's run for the White House, click here to see our special Tracking Rick Perry section. View videos, photos, and stories on the Texas governor's presidential run.