1:45 AM
Las Vegas - Frontrunner Mitt Romney and surging ex pizza firm boss Herman Cain came under fire on Tuesday in a fiery Republican TV debate, but the angriest exchanges came between Romney and struggling Rick Perry.

The Las Vegas debate, possibly the most heated so far, saw Romney and Perry eyeball each other tensely after the Texas governor, battling to regain ground, accused Romney of employing illegal immigrants.

Cain, the blunt-speaking former head of Godfather's Pizza whose poll surge this month rejuvenated the lackluster Republican race, meanwhile clashed with virtually all his rivals over his controversial "9-9-9" tax plans.

Perry, who surged into the lead after joining the race in August but has slumped after a couple of weak debate performances, riled Romney by accusing him of allegedly employing illegal immigrants as gardeners.

"The idea that you stand here before us and talk about, you're strong on illegal immigration, is on its face the height of hypocrisy," an animated 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."

"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," he added.

Negative attack

The debate, hosted in the Venetian hotel and resort on Las Vegas' world-famous Strip, had started with almost all the six other candidates pouring scorn on Cain's 9-9-9 tax plans.

"Herman, I love you, brother, but you don't need to have a big analysis to figure this thing out," said Perry. "Go to New Hampshire where they don't have a sales tax and you're fixing to give them one.

"They're not interested in 9-9-9... I don't think so, Herman. It's not going to fly."

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

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

"Rick Perry had a strategy coming into this debate, to kill Mitt, and he ended up killing himself," said a Romney spokesperson. "Every time Perry resorted to a negative attack, a personal cheap shot, the audience booed.

"So I think his strategy backfired," he added.

But analysts said Perry had done enough to stay in the race - in any case he, like Romney, has a far bigger campaign war-chest and resources than Cain - leaving the race as a three-way contest.

Observers question, however, whether Cain could suffer the same fate as Perry, and Cain acknowledged that sustaining his poll surge is a challenge.

"The way it feels is that you got this big bull's-eye on your back, and it keeps getting bigger," Cain told CNN shortly before the so-called Western Republican Presidential Debate got under way.

Political file

He was criticised for weekend remarks suggesting building an electric fence along the southern US border to keep immigrants out, with a sign warning it "will kill you".

He later said the comments were a joke, but it was not a good move in a state like Nevada - and neighbouring states in the US southwest - with an enormous Latin community.

To cap it off, an old video showing a young Cain singing "Imagine all the Pizza" to the tune of the John Lennon hit "Imagine" did little for his serious political profile.

While the Republicans thrash out their differences out West, back in Washington, Obama's Democratic re-election campaign appeared satisfied at the spectacle of disunity.

"Obamacare," "fence", and "repeal" were the top words of tonight's debate - but #Obama2012 was the real winner," it tweeted.