LAS VEGAS (AP) - Republican presidential contenders attacked upstart Herman Cain's economic plan forcefully as a tax increase waiting to happen Tuesday night, moving swiftly in campaign debate to blunt the former businessman's unlikely rise in the race for the party's nomination.

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Yet old animosities flared anew, as well, as former Massachusetts Gov. Mitt Romney and Texas Gov. Rick Perry 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 clash over immigration.

In on a bow to Nevada voters, none of the contenders said he wanted to open a proposed nuclear waste repository at Yucca Mountain.

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. The candidates engaged each other more directly and sometimes more heatedly than in previous debates.

But Cain's unlikely rise from asterisk in the polls to contender was clearly on the minds of his rivals on stage in a hotel along the Las Vegas Strip.

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.

Cain's proposal is for a 9 percent personal income tax, a 9 percent corporate tax and a 9 percent national sales tax.

The former pizza company CEO is the latest and unlikeliest phenomenon in the race to pick a rival for President Barack Obama. A black man in a party that draws few votes from Africans Americans, he had bumped along with little notice as Romney sought to fend off one fast-rising rival after another.

That all changed in the past few weeks, after Perry burst into the race and then fell back in the polls. However unlikely Cain's rise, Tuesday night's debate made clear that none of his rivals are willing to let him go unchallenged.

"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 they don't have any interest in one," he said, referring to the state that will hold the first primary early next year.