LAS VEGAS (CNN) -- Seven Republican presidential candidates engaged in a sometimes contentious debate Tuesday in Las Vegas, Nevada, as bad blood boiled between front-runners and the surging Herman Cain found his opponents taking aim at his tax plan.
Here are five things we learned from the debate:
Why 9-9-9 was No. 1: A rise in the polls brings more scrutiny, so we knew that Cain's "9-9-9" tax plan would come under attack from his rivals. And we didn't have to wait long for the full frontal assault.
In response to the first question of the debate, from an audience member asking the candidates their "position on replacing the federal income tax with a federal sales tax," the other candidates were quick to pile on and rip apart Cain's much-touted proposal to reform the country's tax code, a plan which has helped the former Godfather's Pizza CEO surge in the polls. The six other candidates sharing the stage with Cain fired away, terming the plan as risky and simplistic.
"Anytime you give the Congress a brand-new tax, it doesn't go away," said Rep. Michele Bachmann of Minnesota.
"Herman's well-meaning, and I love his boldness," said former Sen. Rick Santorum of Pennsylvania. "I give him credit for -- for starting a debate, but it's not good for families."
"Herman, I love you, brother, but let me tell you something, you don't need to have a big analysis to figure this thing out. Go to New Hampshire, where they don't have a sales tax, and you're fixing to give them one," said Texas Gov. Rick Perry. "Herman. It's not going to fly." said Rep. Ron Paul of Texas.
"It's very, very dangerous. And it will raise more revenues," said Rep. Ron Paul of Texas.
"The analysis I did, person by person, return by return, is that middle-income people see higher taxes under your plan," said former Massachusetts Gov. Mitt Romney.
Cain repeatedly defended his plan, saying it would not raise taxes on poor people, and said his opponents were misrepresenting his proposal.
"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," responded Cain.
After the debate, Cain said he was happy with his defense of his plan, but promised a more comprehensive proposal would follow, and he added that the attacks on him are proof that he's now a front-runner in the race for the nomination.
Romney gave as good as he got: We knew Romney would be under the microscope, but the attacks from his fellow candidates seemed a bit more vicious in Las Vegas. Call it a Wild West shootout along the Strip, with Romney the target.
Romney and Santorum got into a shouting match over the Massachusetts health care measure passed when Romney was governor. The plan, known as 'RomneyCare,' has been attacked by fellow Republicans as the genesis for the much-hated national health care law pushed by President Barack Obama and passed by a Democratic-controlled Congress.
"You can't change the facts," shouted Santorum.
"Rick, you had your chance. Let me speak," fired back Romney.
"You're out of time. You're out of time," responded Santorum.
Romney and Perry were involved in another dust-up a few minutes later, with Perry saying on the issue of illegal immigration, "Mitt, you lose all of your standing, from my perspective, because you hired illegals in your home and you knew 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."
A shouting match ensued, with Perry interrupting Romney and Romney firing back "Rick, again, Rick again, I'm speaking," and adding this zinger:
"This has been a tough couple of debates for Rick, and I understand that. And so you're going to get testy."
Romney gave as good as he got, but the fact that "RomneyCare" keeps coming up must be frustrating for him and a concern for his campaign.
Rick Perry finally woke up: Perry's poor debate performances over the past month are a major reason why he's faded in national polling. The Las Vegas debate may be the moment when he started to turn things around.
One thing's for sure, Perry's still got plenty of fight left in him. Perry didn't even wait for the questioning to begin to start going on the attack. He zinged Romney during the candidate self-introductions, saying that he was "an authentic conservative, not a conservative of convenience."
As mentioned, Perry and Romney got into a shouting match over illegal immigration, and they also battled over jobs.
"Mitt, while you were the governor of Massachusetts in that period of time, you were 47th in the nation in job creation. During that same period of time, we created 20 times more jobs," charged Perry.
Ray Sullivan, Perry's communications director, told reporters in the post-debate spin room that the Texas governor wasn't too aggressive in his attacks against Romney, adding that "I think the governor spoke from the heart."
Asked if Perry was presidential in his approach at the debate, Sullivan said "I think he was Rick Perry in his approach and that's a good thing for us."
Eric Ferhnstrom, Romney's senior adviser, disagreed, telling reporters that "Rick Perry had a strategy coming into this debate to kill Mitt, and he ended up killing himself."
Rick Santorum's the fighter: He's low in the polls and he isn't rich with campaign cash, but Santorum's excelling at going on the attack and grabbing attention at the debates.
Santorum hammered Romney over the Massachusetts health care law, saying "You just don't have credibility. Your consultants helped craft Obamacare."
He slammed Cain over the "9-9-9" plan, saying "Herman's well-meaning, and I love his boldness, and it's great. But the fact of the matter is, I mean, reports are now out that 84 percent of Americans would pay more taxes under his plan. That's the analysis."
Plus, he went after both Perry and Romney over the Troubled Asset Relief Program, known as the Wall Street bailout, which is despised by conservatives.
"The problem is -- in the first place, is that several people up here, the, quote, "businesspeople," supported the TARP, supported the bailout. Governors Perry, Romney," said Santorum.
Santorum may be low down in the field, but the good news for him is that there are plenty of debates ahead, including three next month.
Immigration questions not going away: This was supposed to be an election about jobs and the economy, but the topic of illegal immigration continues to rear its head on the campaign trail and in debates.
Perry's poll numbers cratered in September after he defended a law in Texas that offers in-state college tuition breaks to the children of illegal immigrants -- a clear sign that Republican primary voters continue to be deeply concerned about the issue.
Perry tried to throw the issue back at Romney on Tuesday by criticizing his rival for hiring illegal immigrants to do work on his lawn, a controversy uncovered by the Boston Globe during Romney's first presidential bid in 2008.
"Mitt, you lose all of your standing, from my perspective, because you hired illegals in your home and you knew about it for a year," Perry said, turning straight to Romney. "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."
Romney admitted that a landscaping company he hired did employ illegal immigrants, but he said he fired the company when he found out.
Keen on scoring points with GOP voters, the candidates were united in saying that Obama has failed to fix the immigration system.
I think the person who really has a problem with illegal immigration in the country is President Obama," Bachmann said. "It's his uncle and his aunt who are illegal aliens who've been allowed to stay in this country, despite the fact that they're illegal."