"Mitt, you lose all of your standing from my perspective because you hired illegals in your home," Perry said Tuesday in Las Vegas in the last of eight televised GOP debates. "And you knew for -- 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," Perry said.
Romney, a former Massachusetts governor, said: "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."
Perry, Texas' governor, snapped back, "It's time for you to tell the truth."
The two continued to argue, often speaking over each other, with Perry at one point saying Romney once said "there was a magnet of people that will hire illegals ... and you are No. 1 on that list, sir."
As the back-and-forth continued, Romney eventually said, "This has been a tough couple of debates for Rick, and I understand that, and so you're going to get -- you're going to get testy."
The Boston Globe reported in 2006 that Romney hired a company that employed illegal immigrants to do yard work at his suburban Boston home. A year later the newspaper said the yard workers still included illegal immigrants.
Romney said in the debate a lawn care firm had hired the workers and he eventually fired the firm because of the lapses.
Romney said Republicans should welcome legal immigrants.
Former business executive Herman Cain defended his "9-9-9" tax plan, which would replace the existing tax code with a 9 percent national sales tax, a 9 percent income tax and a 9 percent corporate tax.
"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, asserting his campaign's calculations showed the plan would work.
Perry said voters didn't need campaign calculations. "Go to New Hampshire, where they don't have a sales tax, and you're fixing to give them one," he said.
Santorum said Republicans can't trust Romney's promises to repeal the Obama administration's Affordable Care Act because his plan "was the basis for Obamacare."
The Massachusetts plan shares several similarities with the national healthcare overhaul, including mandates that people buy insurance.
Gingrich said the Massachusetts plan called for far more government involvement in the healthcare than Romney claimed.
But Romney countered that Gingrich himself, along with the conservative Heritage Foundation, had been among those who supported the idea of an individual mandate before Romney pushed it in his state.
"That is not true," Gingrich interrupted -- only to acknowledge a moment later that he had, in fact, supported the individual mandate.
Rep. Ron Paul of Texas assailed the others for resisting defense-spending cuts.
"We want to spend more and more, and you can't cut a penny?" he said. "I mean, this is why we're at an impasse. I want to hear somebody up here willing to cut something. Something real."
Rep. Michele Bachmann of Minnesota also participated. Missing from the stage was former Utah Gov. Jon Huntsman Jr., who opted out to support New Hampshire Republicans angry that Nevada moved its caucuses up in the voting schedule to Jan. 14.
Several other candidates, including Gingrich and Cain, said last week they would join Huntsman and boycott Nevada's caucuses if the state party refused to push it back three days as New Hampshire requested.