WASHINGTON (AP) — The Republican presidential candidates tore into each other as never before in their latest debate, mindful that voting starts within 11 weeks and many GOP voters remain up for grabs.

Mitt Romney emerged from the two-hour forum Tuesday night still the person to beat, but he was considerably scratched up on the issues of illegal immigration, health care and jobs.

The feisty faceoff in Las Vegas marked the first time the contenders treated Herman Cain as a serious threat, and they aggressively ripped his 9-9-9 tax plan, perhaps inflicting grave wounds.

And Texas Gov. Rick Perry snapped out of his sleepy debate style, criticizing Romney so vigorously that the two men seemed close to blows at times. Perry was forceful from the start, battling to end his campaign's recent slide and to re-establish himself as the most viable alternative to the former Massachusetts governor.

Cable TV viewers who watched the debate's first 60 minutes and last 10 minutes saw seven contenders make the greatest effort yet to distinguish themselves from one another and expose each other's weaknesses. That left comparatively little time to bash President Barack Obama, but that's something they all agree on anyway.

Some exchanges were personal, almost petty. Romney repeatedly chided Perry and former Pennsylvania Sen. Rick Santorum for talking over him. Perry accused Romney of making false claims, and reminded voters that Romney had employed illegal immigrants to do lawn work a few years ago.

Romney said he instructed the landscape company to get rid of illegal workers.

Perry said it was "the height of hypocrisy" for Romney to criticize the Texan's immigration record. Romney put his hand on the scowling Perry's shoulder and demanded, "Are you just going to keep talking?" Romney said Perry had suffered some poor debates, "so you're going to get testy." It was one of several moments that bordered on condescension.

When Romney said 40 percent of new jobs in Texas lately have gone to illegal immigrants, Perry said heatedly, "That is an absolute falsehood on its face, Mitt."

Rebukes of Cain's 9-9-9 plan dominated the debate's first 15 minutes. Cain, a former pizza company executive, climbed to the top of recent GOP polls by touting his proposal to scrap current income and payroll taxes and replace them with a 9 percent levy on personal and corporate income and a 9 percent national sales tax.

Virtually every rival took a shot at it.

"That's a tax plan, not a jobs plan," said Minnesota Rep. Michele Bachmann. She said a liberal president and Congress might push the sales tax to 90 percent, and consumers would blame vendors.

Rep. Ron Paul of Texas called the plan dangerous and regressive. It's good that nearly half of Americans currently pay no federal income tax, Paul said, adding that he would replace that tax "with nothing."

Several candidates cited a new report by the Tax Policy Center, a Washington think tank, that said Cain's plan would raise taxes on 84 percent of U.S. households.

Cain, on the defensive as never before, said critics were misinterpreting his plan and mixing apples and oranges. Romney turned the phrase against him, saying Americans would be taxed on apples AND oranges because they would pay state and federal sales taxes in most states.

With Cain's 9-9-9 plan treated like road kill, the candidates turned mainly to criticizing Romney and watching him and Perry spar in ways that hinted at real animosity. Santorum, who lost his bid for a third Senate term six years ago, often played the aggressor.

Noting that Romney's Massachusetts health care program had required residents to obtain medical insurance, Santorum said, "Your plan was the basis for Obamacare," the GOP epithet for the Democrats' 2010 health care overhaul.

Romney, sometimes struggling to be heard, repeated his claim that the state plan was meant for Massachusetts alone. Perry joined Santorum in saying Romney at times had signaled that other states should adopt the Massachusetts model.

"The people of Massachusetts like it by about a 3-to-1 margin," Romney said. He added, however, "I didn't get the job done in Massachusetts, and getting the health care costs down in this country is something I think we got to do at the national level."

Former House Speaker Newt Gingrich said it wasn't fair to equate "Romneycare" with "Obamacare." However, he said, "There's a lot of big government behind "Romneycare ... more than your campaign is admitting."

When talk turned to foreign policy, Cain was pressed to explain a CNN interview in which he said he might consider releasing all the terror suspects at the U.S. naval prison at Guantanamo Bay, Cuba, if al-Qaida demanded it as the price of handing over a captured American.

"No, I said that I believe in the philosophy of we don't negotiate with terrorists," Cain said. "I think — I've been saying — I would never agree to letting hostages in Guantanamo Bay go. No, that wasn't the intent at all."

Near the debate's close, Santorum took a swing at Perry and Romney. "I didn't run as a Democrat in Texas when it was popular," he said, alluding to Perry's pre-Republican past. "I didn't run as a liberal in 1994," he said, referring to Romney's unsuccessful bid to oust then-Sen. Ted Kennedy. In that campaign, Romney said he would be a more forceful proponent of gay rights than would Kennedy.

Bachmann got the final word, saying: "The cake is baked. Barack Obama will be a one-term president."

The GOP crowd loved it, of course. Obama certainly faces big re-election hurdles. But Tuesday night's forum pointed to more GOP bloodletting ahead as the rivals lunge for an up-for-grabs nomination.

Romney seems certain to remain the chief target, at least for a while. Perry served notice he's back in the game. And Cain's rough treatment suggested it's possible that Bachmann, Santorum, Gingrich or Paul will have chances to rise in the polls — and then face the consequences.