Las Vegas (CNN) -- Tuesday night was fight night in Las Vegas.





Seven Republican presidential candidates clashed sharply over issues such as illegal immigration, taxes and health care at a presidential debate in Nevada sponsored by CNN and the Western Republican Leadership Conference.





But it was the three Republican front-runners -- former Godfather's Pizza executive Herman Cain, former Massachusetts Gov. Mitt Romney and Texas Gov Rick Perry -- who came under frequent attack.





The long-standing bad blood between Romney and Perry boiled over in the debate's first hour as the two GOP heavyweights traded harsh accusations and showed flashes of anger.





Romney laughed in Perry's face when the Texan criticized his rival for hiring illegal immigrants to work on his lawn, a controversy uncovered by The Boston Globe during Romney's first presidential bid in 2008.





"Mitt, you lose all of your standing, from my perspective, because you hired illegals in your home, and you knew about it for a year," Perry said, turning straight to Romney. "And the idea that you stand here before us and talk about that you're strong on immigration is on its face the height of hypocrisy."





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When Romney tried to respond, Perry refused to let him answer.





"You get 30 seconds," Romney said, looking to moderator Anderson Cooper. "This is the way the rules work here, is that I get 60 seconds and then you get 30 seconds to respond. Right? Anderson?"





Perry interrupted: "And they want to hear you say that you knew you had illegals working at your ... "





"Would you please wait?" a frustrated Romney shot back. "Are you just going to keep talking?"





"Yes, sir," Perry said.





An exasperated Romney tried to rise above the squabble.





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





Romney fired back that Perry failed to secure the Texas border and again raised questions about his support for a law that granted in-state college tuition to the children of illegal immigrants.





"Texas has had 60% increase in illegal immigrants in Texas," Romney charged. "If there's someone who has a record as governor with regards to illegal immigration that doesn't stand up to muster, it's you, not me."





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Perry's debate performance was, by far, his feistiest to date.





After surging to the top of Republican polls on joining the presidential race in August, Perry saw his standing slip among GOP voters after a series of lackluster debate showings.





But it was not clear as the debate concluded Tuesday whether Perry's aggressive tack would hurt him or help him.





A seemingly Romney-friendly audience in Las Vegas booed when Perry turned up the heat, and after the debate, one of Romney's senior advisers claimed that Perry only hurt himself by going so negative.





"Rick Perry came in to kill Mitt, but he killed himself," Romney adviser Eric Fehrnstrom said in the post-debate spin room.





Ray Sullivan, Perry's communications director, claimed victory and called Romney a "slick guy" who will "change positions on a dime."





"He is back to running again," Sullivan said of Perry. "This is a campaign that is in for the long haul."





Cain, meanwhile, was at the center of a pile-on as the debate began.





His GOP rivals took turns picking apart his "9-9-9" plan to reform the United States tax code.





The plan would throw out the current tax system and implement a 9% corporate income tax rate, a 9% income tax rate and a 9% national sales tax.





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The other six Republicans on stage
Seven of the top GOP presidential candidates faced off where? Who are the front runners? Romney and Perry traded what? How many GOP candidates faced off in Vegas?
Nevada Herman Cain, harsh accusations Seven
question: Seven of the top GOP presidential candidates faced off where?, answer: Nevada | question: Who are the front runners?, answer: Herman Cain, | question: Romney and Perry traded what?, answer: harsh accusations | question: How many GOP candidates faced off in Vegas?, answer: Seven