Republican presidential rivals Mitt Romney and Rick Perry were involved in a verbal spat on Tuesday at a CNN-sponsored debate in Las Vegas. But despite disagreements on immigration and healthcare, the two main contenders united to criticize fellow-candidate Herman Cain's 9-9-9 tax plan

Seven Republican candidates still in the running - Romney, Perry, Cain, Reps. Michele Bachmann of Minnesota and Ron Paul of Texas, former Pennsylvania Sen. Rick Santorum and former House Speaker Newt Gingrich met at the Venetian hotel in Las Vegas. Former Utah Gov. John Hunstman was missing.

Romney, the former Massachusetts governor already considered the strongest Republican contender, was critical of both Cain and Texas Gov. Perry, currently his two strongest rivals.

Rick, I don't think that I've ever hired an illegal in my life. And so I'm - I'm looking forward to finding your facts on that, said Romney lightly, when Perry accused him of hiring illegal immigrants to cut his lawn.

It's time for you to tell the truth, Romney continued sharply. Texas has had a 60 percent increase in illegal immigrants in Texas. 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.

I suggest that if you want to become president of the United States, you have got to let both people speak, said an agitated Romney, after being repeatedly interrupted by Perry.

Romney also came under attack from Santorum, for his state health plan, which was a model for the healthcare bill implemented by President Barack Obama last year.

You just don't have credibility, Mitt, when it comes to repealing Obamacare; your plan was the basis for Obamacare; your consultants helped Obama craft Obamacare and to say that you would repeal it, you have no track record on that that we can trust you that you are going to do that, said Santorum.

Supporters of Cain also believe Romney is not conservative enough to lead the Republican cause.

Romney, who appeared to be a far more confident and refined debater than he was during his 2008 presidential run, defended himself and his state health plan by saying it was appropriate for his state, Massachusetts, but not for the nation.

Romney's religion - he is a Mormon - was also discussed but he again defended his position, saying the idea that we should choose people, based on their religion, for public office is what I find to be most troubling.

Perry and Romney on Cain's Tax Plan

Perry also criticized Cain's tax plan.

Herman, I love you, brother, but you don't have to have a big analysis to figure this out. ... You want 9 per cent on a new home, 9 per cent on your Social Security. I don't think so, Herman. It's not going to fly, said Perry, according to a report by Fox News.

Cain, however, maintained that critics misunderstood his plan and were mixing apples and oranges.

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, he said, They want to continue to be able to manipulate the American people with a 10-million-word mess.

Romney, at this point, fired off at Cain: Fine. 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.

Michele Bachmann added that Cain's plan would establish a value-added tax because at every step and stage of production, you'd be taxing that item 9 percent on the profit.

Cain, whose 9-9-9 plan has come under major scrutiny since he announced it, urged Americans to judge the plan for themselves.