ABC News’ Michael Falcone and Shushannah Walshe report:

LAS VEGAS — At the feistiest debate of the presidential campaign, Herman Cain came under fire, Rick Perry came prepared and Mitt Romney came to defend his status as the candidate to beat in the fight for the Republican presidential nomination.

In the midst of a particularly volatile period in the primary cycle, the candidates threw some of their sharpest elbows yet. They got angry and at one point, they even got physical.

It was Perry’s fifth debate as a presidential candidate and after stumbling in some of his earlier outings and fading into the background at last week’s showdown in New Hampshire, he roared back onto the scene on Tuesday night, unveiling a fresh attack on Romney.

At the CNN-Western Republican Leadership Conference debate in Las Vegas, Perry attempted to turn an issue that has been seen as an area of weakness for the Texas governor — illegal immigration — into an instrument of destruction against his rival.

“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. “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.”

Romney fiercely denied the accusation, which is based on a five-year-old news report that the former Massachusetts governor had employed two illegal immigrants to do yard work.

“Rick, I don’t think I’ve ever hired an illegal in my life,” Romney shot back, ”and so I’m afraid — I’m looking forward to finding your facts on that.”

“Well, I’ll tell you what the facts are,” Perry began to say, before being interrupted by a fuming Romney who moments later laid a hand on his opponent’s shoulder.

“I’m speaking, I’m speaking, I’m speaking,” shouted Romney, who later scolded the Texas governor: “I’m running for office, for Pete’s sake, I can’t have illegals.”

Watch the exchange here:

The testy moment reflected deepening tensions between the Romney and Perry campaigns with both sides contesting the GOP presidential nomination with renewed urgency as the clock ticks down to the nation’s early primaries and caucuses.

But in recent days, Romney and Perry have had to make way for a new contender — businessman Herman Cain — who has taken the race by storm with his so-called 9-9-9 economic plan. That proposal came under withering criticism from rival candidates tonight.

“The reason that my plan — the reason that our plan is being attacked so much is because lobbyists, accountants, politicians, they don’t want to throw out the current tax code and put in something that’s simple and fair.  They want to continue to be able to manipulate the American people with a 10 million-word mess,” Cain said. “Let’s throw out the 10 million-word mess and put in our plan, which will liberate the American workers and liberate American businesses.”

Former Pennsylvania Sen. Rick Santorum said he admired Cain’s “boldness,” but, “the fact of the matter is, I mean, reports are now out that 84 percent of Americans would pay more taxes under his plan.”

Minnesota Rep. Michele Bachmann found reason to take issue with Cain as well.

“One thing I know about Congress, being a member of Congress for five years, is that any time you give the Congress a brand new tax, it doesn’t go away,” she said.

And Perry dismissed it, too.

“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.  Go to New Hampshire, where they don’t have a sales tax, and you’re fixing to give them one,” he said. “They’re not interested in 9-9-9.”

Cain accused his detractors of “mixing apples and oranges.”

“Unfortunately, none of my distinguished colleagues who have attacked me up here tonight understand the plan,” said the former Godfather’s Pizza CEO, who has recently passed Perry in most national polls has even crept past Romney in a few.

“Will the people in Nevada not have to pay Nevada sales tax and in addition pay the 9 percent tax?” Romney asked Cain, who replied “no, no, no, no” and again accused Romney of mixing up his fruits.

“Fine,” Romney said. “And I’m going to be getting a bushel basket that has apples and oranges in it because I’ve got to pay both taxes, and the people in Nevada don’t want to pay both taxes.”

And while the back-and-forth over the economic effects of Cain’s plan dominated the early part of Tuesday night’s exchange, held in an auditorium just off the Las Vegas strip, Romney also found himself on the receiving end of attacks over the health care law he supported as governor of Massachusetts.

Santorum accused Romney of passing his own version of “Obamacare.”

“What you did is exactly what Barack Obama did,” Santorum said, “focused on the wrong problem.”

Seven of the leading contenders for the GOP presidential nomination took the stage Tuesday night, but one was absent — former Utah Gov. Jon Huntsman. He decided to boycott the debate to protest Nevada’s decision to hold its nominating caucuses on Jan. 14, putting it on a collision course with New Hampshire, a state that has become crucial to Huntsman’s presidential hopes.

“I was invited to a game show in Las Vegas, with lots of talking points and buzzers,” Huntsman said Tuesday night before his rivals took the stage in Las Vegas. “What happens in Vegas stays in Vegas right? What happens in New Hampshire impacts the world.”

Though most of the debate focused on policy issues, at one point the moderator, CNN’s Anderson Cooper, raised the sensitive topic of religion, referring to the comments of a evangelical pastor who endorsed Perry and called Romney’s religion — Mormonism — “a cult.”

Santorum citing his own Catholicism said voters “should pay attention to the candidate’s values, what the candidate stands for.”

“It’s a legitimate thing to look at as to what the tenets and teachings of that faith are with respect to how you live your life and — and how you would govern this country,” Santorum said referring to a candidate’s religion. ”With respect to what is the road to salvation, that’s a whole different story.  That’s not applicable to what — what the role is of being the president or a senator or any other job.”

Former House Speaker Newt Gingrich agreed telling the crowd that “none of us should rush in judgment of others in the way in which they approach God.”

“I, frankly, would be really worried if somebody assured me that nothing in their faith would affect their judgments, because then I’d wonder, where’s your judgment — how can you have judgment if you have no faith?  And how can I trust you with power if you don’t pray?,” Gingrich asked.

After the pastor, Robert Jeffress, who runs a Texas mega-church, made the comments at a conference of cultural conservatives in Washington, DC earlier this month, Romney has called on Perry to “repudiate” the statement. So far Perry has declined to take that step, and that did not change Tuesday night.

“I didn’t agree with that individual’s statement,” Perry said, adding that Pastor Jeffresss had a right to his opinion.

“The idea that we should not have our freedom of religion to be taken away by any means, but we also are a country that is free to express our opinions.  That individual expressed an opinion.  I didn’t agree with it, Mitt, and I said so,” Perry told Romney.

In response, Romney said what he found “most troubling” with what Jeffress said was that “we should choose people based upon their religion for public office.”

“That we would not choose people who represent us in government based upon their religion, that this would be a nation that recognized and respected other faiths, where there’s a plurality of faiths, where there was tolerance for other people and faiths,” Romney said. “The concept that we select people based on the church or the synagogue they go to, I think, is a very dangerous and — and enormous departure from the principles of our Constitution.”

With “Occupy Wall Street” protests continuing to take place across the country, Cain was asked about his comments two weeks ago that if the protestors don’t have a job — “blame yourself.”

Cain said he stood by his comments because the protestors should instead be “in front of the White House taking out their frustration.”

“They might be frustrated with Wall Street and the bankers, but they’re directing their anger at the wrong place,” he said. ”Wall Street didn’t put in failed economic policies.  Wall Street didn’t spend a trillion dollars that didn’t do any good.  Wall Street isn’t going around the country trying to sell another $450 billion.”

The libertarian-minded Congressman Ron Paul of Texas took on Cain directly saying the businessman “blamed the victims” when both Wall Street and the Federal Reserve were really at the root of the country’s fiscal woes.

“The middle class got stuck. They got stuck. They lost their jobs, and they lost their houses. If you had to give money out, you should have given it to people who were losing their mortgages, not to the banks,” Paul said to Cain.

Israel’s prisoner swap on Tuesday in which a soldier captured and held for five years by the Palestinian terrorist group, Hamas, was traded for hundreds of Palestinian prisoners serving life sentences in connection to terror related crimes, also became a contentious subject on Tuesday night. Earlier in the day, CNN anchor Wolf Blitzer asked Cain if he could ever see himself “authorizing that kind of transfer” with those held at Guantanamo Bay.

Cain’s response: “I could, but what I could do is make sure that I got all of the information.”

His rivals immediately pounced, stressing they would never negotiate with terrorists.

“For any candidate to say that they would release the prisoners at Guantanamo in exchange for a hostage would be absolutely contrary to the historical nature of the United States and what we do in our policy. That’s naïve; we cannot do that,” Bachmann said. “We don’t negotiate.”

Cain steped back from his original comments at the debate, saying that he believes “in the philosophy of we don’t negotiate with terrorists” adding he would “never agree to letting hostages in Guantanamo Bay go.”

During the same segment, the candidates were asked about foreign aid. Perry received loud applause from the crowd when he said that it is time for the country to have a “very serious conversation about defunding the United Nations.”

The debate did not end without some time for local issues. An audience member asked the candidates about Nevada’s sky-high foreclosure rate and Bachmann insisted that she was the candidate who would help them hold on to their homes.

“President Obama has failed you on this issue of housing and foreclosures.  I will not fail you on this issue.  I will turn this country around. We will turn the economy around.  We will create jobs.  That’s how you hold on to your house,” Bachmann said looking straight into the camera. “Hold on, moms out there.  It’s not too late.”

But the night was marked more by the areas of differences between the candidates than their similarities. Gingrich, who has repeatedly criticized the media’s coverage of the primary race, did so again on Tuesday. And he also issued a warning to his fellow candidates.

“Let me just point out for a second,” Gingrich said, “that maximizing bickering is probably not the road to the White House.”

ABC News’ Amy Walter, Arlette Saenz, Sarah Kunin, Susan Archer and Emily Friedman contributed reporting.