Rick Perry and Mitt Romney squared off on stage at the Sands Expo Center in Vegas. (Getty)

Sparks finally flew in the Republican primary as Texas Governor Rick Perry and Mitt Romney got into a tense shouting match during Monday night's CNN/Western Republican Leadership Conference debate in Las Vegas. The pair clashed over jobs, religion and Governor Perry's accusation that Mr. Romney had illegal immigrants mow his lawn.

Things got nasty during a conversation on health care. Governor Perry said his home state is affected by large numbers of illegal immigrants and said Mr. Romney hired illegal aliens.

"You hired illegals in your home and you knew about if for over a year. ... The idea that you can sit here and talk about any of us having an immigration issue is beyond me," Governor Perry said.

"I don't think I've ever hired an illegal in my life, so I look forward to finding your facts on this," Mr. Romney retorted.

Governor Perry's attack on Mr. Romney referred to a 2006 story from the Boston Globe that accused Mr. Romney of using a landscaping company that employed illegal immigrants to do work at his 2-and-a-half acre Massachusetts estate. Mr. Romney has been dogged by the allegations ever since. He quickly clarified his statement that he'd never "hired an illegal."

"We hired a lawn company to mow our lawn and they had illegal immigrants that were working there and we let them go," Mr. Romney said.

Governor Perry and Mr. Romney shouted over each other and repeatedly interrupted each other as they discussed the issue.

"I suggest that if you want to become President of the United States, you got to let both people speak," Mr. Romney said.

Mr. Romney mocked Governor Perry's attacks on him as a desperate attempt to save his sinking poll numbers.

"This has been a tough couple of debates for Rick and I understand that," Mr. Romney said.

In addition to their faceoff over Mr. Romney's lawn, the pair argued about their jobs records as governors of their respective states.

"The fact is, Texas has led the nation in job creation. … That's what Americans are looking for. … You failed as the Governor of Massachusetts," Governor Perry said to Mr. Romney.

"With regards to track record, in the past Governor, you were chairman of Al Gore's campaign," Mr. Romney retorted.

Governor Perry tried to bring up Mr. Romney's lawn again during the discussion on jobs, but he was cut off by the debate's moderator, Anderson Cooper.

"I think we've been down that road," Mr. Cooper said.

Near the end of the debate, Mr. Cooper brought up another issue that has been a source of conflict for Governor Perry and Mr. Romney--comments by, Dr. Robert Jeffress, who criticized Mr. Romney's Mormon religion as "a cult" while introducing Governor Perry at an event. Mr. Cooper asked Mr. Romney about his demand that Governor Perry "repudiate" Jeffress.

"That idea that we should choose people based upon their religion for public office is what I find the most troubling," Mr. Romney said.

Governor Perry attempted to distance himself from Jeffress' comments without entirely apologizing for them or backing down. He said he supported freedom of religion, but also "freedom to express our opinion."

"I did not agree with the--Pastor Jeffress' remarks. I don't agree with them, I can't apologize any more than that," Governor Perry said.

With Governor Perry's showdown with Mr. Romney stealing the show at the debate, the other Republican candidates were pushed into the background. As Mr. Cooper attempted to bring the night to a close, Michele Bachmann interrupted him to inject a note of Republican solidarity--and promote her campaign.

"The good news is, the cake is baked, Barack Obama will be a one term President. ... We need to listen to Ronald Reagan who said, 'No pastels, bold colors.' … I am the most different candidate from Barack Obama," Ms. Bachmann said.