LAS VEGAS (AP) ― Republican presidential contenders piled on the two front runners in a feisty debate Tuesday night, attacking businessman Herman Cain’s economic plan as a tax increase and former Massachusetts Gov. Mitt Romney over his health care law.

Rep. Michele Bachmann of Minnesota led the assault Tuesday night, saying Cain’s call for a 9 percent federal sales tax would only be the beginning, with the rate rising later.

Former Sen. Rick Santorum cited one analysis that found that taxes would go up for 84 percent of American households under the plan.

Texas Gov. Rick Perry and Santorum tag-teamed Romney over Massachusetts’ health care law that Democrats used as a model for their national health care plan.

Perry said Romney is being dishonest about his record on the law, while Santorum was more direct: “You just don’t have credibility, Mitt.”

Romney defended his plan as right for his state but wrong for the nation.

Cain, for his part, insisted the charges about his tax plan were untrue and that he was being criticized because lobbyists, accountants and others stood to benefit from the current tax code.

Cain has never held public office, but has built up a following among conservative activists as a radio talk show host and motivational speaker. That background has boosted his popularity with his poll numbers having jumped to put him in a dead heat with the presumed front-runner, former Massachusetts Gov. Mitt Romney.

All are vying for the Republican nomination to challenge Democratic President Barack Obama in 2012, and many undecided voters have been watching the debates to evaluate the candidates.

Much of the focus Tuesday has been on Cain’s catchy “9-9-9” tax overhaul plan which he’s made the centerpiece of his campaign. The plan would scrap the current tax code and replace it with a 9 percent tax on personal income and corporations as well as a new 9 percent national sales tax.

The backdrop for the debate was the state’s the 13.4 percent unemployment rate, the country’s highest. The formerly fast-growing neighborhoods north of the glittering Las Vegas strip are now wracked by foreclosures.

Texas Rep. Ron Paul and former House of Representatives Speaker Newt Gingrich of Georgia are also taking part in the debate, the fifth in six weeks, and the last scheduled for nearly a month in a race that is fluid in more than one way.

While polls chart a series of rises and falls for various contenders ― Romney remaining at or near the top ― the schedule is far from set.

It was Perry who instigated the confrontation over immigration, saying that Romney had no credentials on the issue because he had once hired an illegal worker, the “height of hypocrisy.”

Romney denied the charge, saying he had hired a company to mow his lawn and did not know that it had an illegal immigrant on its payroll. Bachmann, Santorum and Paul all sidestepped the question.

Cain found himself on the defensive on two other issues during the two-hour debate. He apologized for earlier remarks about building an electric fence on the Mexico border that could kill people trying to cross illegally.

And he said he wouldn’t be willing to negotiate with terrorists, even though he had appeared to suggest he might be in an interview earlier in the day.

Former Utah Gov. Jon Huntsman skipped this debate, saying he was boycotting the Nevada caucuses in a dispute over the primary and caucus calendar. He is campaigning exclusively in New Hampshire in hopes of a victory that can move him into the thick of the race.

Not only Republicans, but Obama was also critical of Cain’s economic plan during the day.

In an interview with ABC News, Obama said it would be a “huge burden” on middle-class and working families.