Former pizza firm boss Herman Cain clashed with his Republican rivals over his controversial "9-9-9" tax plan, as he battled to sustain a surprise poll surge in his party's presidential race.
In a noticeably testy TV debate, the plain-speaking Cain repeatedly insisted critics had not understood his tax plan, in particular its proposed new nine percent federal sales tax.
"Herman, I love you, brother, but you don't need to have a big analysis to figure this thing out," said Texas Governor Rick Perry, the former frontrunner who is struggling to claw his way back after a poll slump.
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"Go to New Hampshire where they don't have a sales tax and you're fixing to give them one. They're not interested in 9-9-9... I don't think so, Herman. It's not going to fly."
The debate, hosted at The Venetian hotel and casino in Las Vegas, saw some particularly tetchy exchanges between Perry and former Massachusetts governor Mitt Romney, the only candidate to have remained a frontrunner for long.
"You have a problem with allowing someone to finish speaking," a clearly angry Romney told Perry, standing directly to his left, after at least two exchanges in which the men spoke over each other.
"I suggest that if you want to become president of the United States, then you need to let both people speak," he added.
While Romney remains the long-term frontrunner, Cain's surge in the opinion polls has rejuvenated the lackluster race to choose the Republican nominee to take on President Barack Obama next year.
On the eve of the debate, a CNN/ORC poll gave Romney 26 percent, with Cain only one point behind at 25 percent, within the margin of error and well ahead of the next-placed Texas governor Rick Perry.
And Cain led the pack on likability and the candidate most likely to get the economy moving again. But he did not pass the key "electability" test with most voters, with Romney the clear winner.
Some 51 percent of respondents said Romney is most likely to win the nomination, while just 18 percent said that of Cain and 14 percent thought Perry would win.
Observers question whether Cain could suffer the same fate as Perry, who surged into the lead after joining the race in August, only to fall behind after a couple of lackluster debate performances.
"The way it feels is that you got this big bull's-eye on your back, and it keeps getting bigger," Cain told CNN shortly before the latest TV debate got underway.
Hours before the evening debate, questions were raised over his finances, after reports that Cain used his own campaign funds to buy copies of "This is Herman Cain! My Journey to the White House."
Cain was also criticized for weekend remarks in which he suggested building an electric fence along the southern US border with Mexico to keep immigrants out that would bear a sign warning it "will kill you."
He admitted the comments were a joke, but it was not a good move in a state like Nevada -- and neighboring states in the US southwest -- with an enormous Latin community.
To cap it off, an old video showing a young Cain singing "Imagine all the pizza" to the tune of the John Lennon hit "Imagine" did little for his serious political profile.
In the very latest blow for his campaign, US media reported Federal Election Commission records and found that Cain paid his company $36,511 for his own book.
Cain had denied the purchases violated election rules, according to The Washington Post.
