The poll surge of Cain, a former pizza company boss, has rejuvenated the lacklustre Republican race [Reuters]

Republican presidential contenders in the US have had held a fiery debate which saw frontrunner Mitt Romney and Herman Cain, the former head of Godfather's Pizza, come under fire from their rivals.

Cain, whose poll surge this month has rejuvenated the lacklustre Republican race, clashed with virtually all the other contenders over his controversial "9-9-9" tax plans, which he has made the centrepiece of his campaign.

The plan would scrap the current tax code and replace it with a nine per cent levy on personal and corporate income as well as a new nine per cent national sales tax.

"Herman, I love you, brother, but you don't need to have a big analysis to figure this thing out," said Texas Governor Rick Perry during the debate in Las Vegas.

"Go to New Hampshire where they don't have a sales tax and you're fixing to give them one."

Michele Bachmann, the Minnesota Republican, who also enjoyed a quick rise after a strong debate performance in New Hampshire, and a quick fall when national support did not develop, went on the attack, calling Cain's plan a value added tax that would cause prices to snowball as products moved up the production line.

'Testy'

The angriest exchanges in the debate saw Romney and Perry eyeball each other tensely after the Texas governor, battling to regain ground, accused Romney. the former governor of Massachusetts, of employing illegal immigrants.

Perry, who surged into the lead after joining the race in August, but has slumped after a couple of weak debate performances, accused Romney of being dishonest.

"The idea that you stand here before us and talk about, you are strong on illegal immigration, is on its face the height of hypocrisy," Perry said.

Romney, trying to respond but repeatedly interrupted by Perry standing just to his left, retorted: "This has been a tough couple of debates for Rick and I understand that, so you're going to get testy."

He denied Perry's charge, saying he had hired a company to mow his lawn and did not know that it had illegal immigrants on its payroll.

"You have a problem with allowing someone to finish speaking ... I suggest that if you want to become president of the United States, then you need to let both people speak," Romney said.

At the end of the evening, Romney's camp said that Perry, desperate to get himself back into the race, had misfired.

'Defunding UN'

The debate, the fifth in six weeks, ranged over familiar and contentious territory - from health care reform to the economy and energy - with foreign policy taking a secondary role.

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 US is automatically a citizen.

He also said it was "time to have a very serious discussion about defunding the United Nations".

Libertarian-leaning Texan Ron Paul said US troops should be withdrawn from Korea, where they have been stationed for more than 50 years, and foreign aid to Israel cut.

Jon Huntsman, the former Utah governor, skipped this debate, saying he was boycotting the Nevada caucuses in a dispute over the primary and caucus calendar.

While polls chart a series of rises and falls for various contenders, Romney has consistently remained at or near the top but with fairly static numbers.

On the eve of the debate, a CNN/ORC poll gave Romney 26 per cent, with Cain only one point behind on 25 per cent, both of them well ahead of Perry at 13 per cent.

Source: Agencies