GOP front-runner Herman Cain opens up to Piers Morgan about race, cancer, gay marriage and more. Watch the interview on "Piers Morgan Tonight," Wednesday night at 9 ET/PT on CNN.
Las Vegas (CNN) -- The Republican presidential field's Mr. Nice, Herman Cain, vowed to go on the attack early Wednesday morning, after an often sharp-tempered debate among the seven rivals.
"They're getting on my last nerve," he told CNN's Carol Costello after opponents criticized his "9-9-9" tax reform plan.
Their attacks on him suggest "they don't have a real convincing plan" and that they had not studied his thoroughly, he said after the debate.
"Show me your analysis that shows it won't work," said Cain, who is running near the top of the field in several recent polls.
Cain, a former Godfather's Pizza executive, may not help himself by adopting the combative stance used by former Massachusetts Gov. Mitt Romney and Texas Gov. Rick Perry.
Two undecided debate watchers complained about the tone of the candidates, saying they wanted substance, not attacks.
"I was a little tired of it," said Tracy Smith. "I wanted to hear more about the issues... instead of sparring back and forth."
Bill Sidhu agreed, asking: "Where do the candidates stand on the real issues?"
Neither was committed to a candidate yet, they said, though Smith said she was interested in hearing more from Cain and Romney, and described herself as "a little over Rick Perry at this point, but it's still a very fluid situation."
CNN senior political contributor Ron Brownstein, on the other hand, said the debate was the beginning of Perry recovering himself.
The seven candidates clashed sharply over issues such as illegal immigration, taxes and health care at the debate in Nevada sponsored by CNN and the Western Republican Leadership Conference.
The long-standing bad blood between Romney and Perry boiled over in the debate's first hour as the two GOP heavyweights traded harsh accusations and showed flashes of anger.
Romney laughed in Perry's face when the Texan criticized his rival for hiring illegal immigrants to work on his lawn, a controversy uncovered by The Boston Globe during Romney's first presidential bid in 2008.
Romney fired back that Perry failed to secure the Texas border and again raised questions about his support for a law that granted in-state college tuition to the children of illegal immigrants.
Perry's debate performance was, by far, his feistiest to date.
After surging to the top of Republican polls on joining the presidential race in August, Perry saw his standing slip among GOP voters after a series of lackluster debate showings.
Cain, meanwhile, was at the center of a pile-on as the debate began.
His GOP rivals took turns picking apart his "9-9-9" tax plan.
The plan would throw out the current tax system and implement a 9% corporate income tax rate, a 9% income tax rate and a 9% national sales tax.
The other six Republicans on stage -- who have watched Cain soar to the top of the polls in recent weeks thanks to his straightforward message and outsider image -- were united in calling the tax plan simplistic and risky.
"There are much more complexities than Herman lets on," said former House Speaker Newt Gingrich.
Texas Rep. Ron Paul called the plan "dangerous" and said it would increase taxes on low-income citizens.
Perry told Cain that voters "are not interested in 9-9-9, what they are interested in is flatter and fairer."
Romney said Cain's plan would add federal taxes on top of state taxes.
Cain said his plan would not raise taxes on poor people, and said his opponents were misrepresenting his idea.
They were joined on the stage in Las Vegas by former Pennsylvania Sen. Rick Santorum and Minnesota Rep. Michele Bachmann, who also attacked the Cain plan.
Former Utah Gov. Jon Huntsman chose to boycott the debate because of a dispute between Nevada and New Hampshire over the GOP primary calendar.
Huntsman is staking his campaign on a win in New Hampshire and held a town hall there Tuesday night instead.
CNN Political Reporter Peter Hamby and CNN's Kevin Bohn contributed to this report.