LAS VEGAS - Republican presidential candidates brawled Tuesday over Herman Cain's 9-9-9 tax plan and Mitt Romney's record on illegal immigration and health care, as rivals hammered the two top-tier contenders in the liveliest GOP clash of the 2012 campaign.

The sometimes-angry clash at the Venetian Hotel Resort Casino featured Texas Gov. Rick Perry accusing Romney, the former Massachusetts governor, of "the height of hypocrisy" on immigration. Romney scolded Perry for interrupting him and said Perry was "testy." And candidates were sometimes difficult to understand as they talked over one another.

Cain, the Georgia businessman who surged to the top tier of national polls in recent weeks, was under fire for his plan to scrap the federal tax code and replace it with 9 percent taxes on individuals, businesses and sales.

"Middle-income people see higher taxes under your plan," said Romney, one of several candidates to pile on Cain from the opening minutes of the two-hour debate.

"Reports are now out that 84 percent of Americans would pay more under his plan," said former Sen. Rick Santorum of Pennsylvania. "You're talking about major increases in taxes on people."

Rep. Michele Bachmann of Minnesota questioned whether the sales tax would become a "value-added" levy that would affect multiple layers of production and stunt economic growth.

Perry, who needed a strong showing, joined the fray.

"You don't need to have a big analysis to figure this thing out," Perry said. The Cain tax would add a 9 percent sales tax in states such as Nevada, which already has a sales tax rather than an income tax, and in New Hampshire, where voters are accustomed to paying no sales tax.

Cain brushed aside the torrent of criticism. "It does not raise taxes on those making the least," he said. "That simply is not true."

But a new analysis from the Tax Policy Center, released late Tuesday afternoon, said the Cain plan would raise taxes on all taxpayers making less than $200,000, about 84 percent, while cutting taxes for higher incomes.

Also sparking a lively battle was immigration, a particularly sensitive issue in Nevada and the states around it.

Perry, defending his efforts to monitor the Texas-Mexico border, went after Romney.

"Mitt," he said, "you lose all of your standing, from my perspective, because you hired illegals in your home and you knew about it for a year. 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."

"Rick," Romney shot back, "I don't think I've ever hired an illegal in my life."

Perry interrupted. Romney tried to speak. "I'm speaking," Romney repeated three times.

Romney for several years used a lawn service at his home that employed illegal immigrants.

Romney also found himself on the defensive Tuesday on another familiar topic: his support of a Massachusetts law that requires nearly everyone in the state to obtain health care coverage, considered a model for the 2010 federal health care law that Republicans loathe.

Romney has repeatedly said he didn't intend for the federal government to copy Massachusetts.

The leading candidates arrived at the forum with very different goals. Romney was hoping for another sure-footed performance in hopes of tamping down persistent doubts within the party.

Perry, whom Romney's advisers still consider his most formidable competitor, was seeking to regain the stature he had briefly as a top-tier candidate. That stature has suffered after a series of mediocre debate performances.

Bachmann, who also had a moment in the political sun before fading into the shadows after Perry entered the race, was also seeking to re-emerge, although the fact that she was largely ignored by her aggressive rivals suggests they no longer consider her a threat.

Rounding out the group was Rep. Ron Paul of Texas, who assailed the others for resisting cuts in defense spending, saying: "We want to spend more and more, and you can't cut a penny? I want to hear somebody up here willing to cut something. Something real."

Missing from the stage was former Utah Gov. Jon Huntsman, who announced last week that he would boycott to register his protest to Nevada's decision to move its caucuses to Jan. 14, a move that violates Republican Party rules.

The New York Times and Washington Post contributed to this report.