The three-pronged plan, which comprises a 9 percent national sales tax, a 9 percent income tax and a 9 percent corporate tax, was a hot topic at the most recent Republican debate on Tuesday, as Cain's election opponents sought to discredit the idea. 

“Eliminating taxes on capital gains and dividends and combining that with huge rate cuts in both corporate and income taxes would create an unparalleled economic boom," Chocola said. "9-9-9 also eliminates the regulatory and compliance costs from the current tax code that suck billions out of the economy each year."

Earlier this month, Chocola had praised Cain and said he was rising in the polls because "his clear message of limited government and economic freedom is resonating." 

Club for Growth's endorsement clashed with a slew of opposition that started at Tuesday night's GOP debate and extended from that stage to conservative groups that back changes to the tax code. 

“When you take the 9-9-9 plan and you turn it upside down, I think the devil’s in the details,” Minnesota Rep. Michele BachmannMichele Marie BachmannYes, condemn Roseanne, but ignoring others is true hypocrisy Bachmann won't run for Franken's Senate seat because she did not hear a 'call from God' Billboard from ‘God’ tells Michele Bachmann not to run for Senate MORE said Tuesday night. 

Rick Santorum, a former senator from Pennsylvania, called the plan “naïve.”

And former Massachusetts Gov. Mitt Romney told Cain that “simple answers are always very helpful, but oftentimes inadequate.”

On Thursday, Grover Norquist, who heads the anti-tax advocacy group Americans for Tax Reform, came out Thursday against the tax overhaul proposal.

He told MSNBC that the plan is "like having three needles in your arm taking blood out. It's much more dangerous than just one."  

The criticism didn't stop Cain from vaulting to the top of the polls — in the recent NBC/WSJ poll, Cain leads with 27 percent of support from Republican voters, followed by Romney at 23 percent and Perry at 16.

Meanwhile, liberals have argued that the plan would be regressive — raising the taxes of the poor and middle classes, while lowering those of the rich — and inadequate to fund the federal government.

Conservatives, including Norquist, are concerned about a national sales tax that could be easily raised and create a greater tax burden. 

"With the caveat that I understand this cry of rage that people have about the present structure and wanting to do something radically different, I'm much more comfortable taking the present mess and chipping away at it like an ice sculpture to get it down to what you want," Norquist said. 

In an assessment this week, Politifact asked several accountants to run the numbers — they determined that Cain's plan could benefit single taxpayers making $50,000 a year compared with the current, more complex tax code. 

But those savings gradually decline as income levels drop, they found. 

For taxpayers with dependents the results are worse because of how the current tax code treats married couples with children. Cain's plan could cost those families thousands more each year. 

Still, Chocola argued that Cain's plan is better than the current system. 

“Those who argue against the sales-tax component of 9-9-9 really miss the mark,” he said. "Herman Cain’s proposal might not be the perfect plan, but it is a truly revolutionary tax reform that would amount to a massive job-creating tax cut on investments, savings and income.

"Instead of tearing down ideas that would create economic growth and jobs, the other Republican presidential candidates should produce their own plans to achieve a flatter and more growth-oriented tax code.”