July 31, 2007  -- Former Sen. Fred Thompson, R-Tenn., appears to have flip flopped on his pledge to sign federal legislation replacing all federal taxes with a 23 percent sales tax, according to an unedited FairTax.org video reviewed by ABC News.

"He has not taken this pledge," Thompson spokesperson Linda Rozett told ABC News.

The Thompson camp's denial appears to be contradicted, however, by an unedited FairTax.org video in which Thompson is asked, "Senator, if the House and Senate pass the 'Fair Tax' bill do you feel right now that you would sign it?"

Thompson replies to the question by saying, "Yeah, absolutely."

"Fred Thompson may have spoken without thinking. But the tape is accurate," said FairTax.org spokesman Ken Hoagland.

The Democratic National Committee seized on the all-but-announced candidate's apparent contradiction with his own staff, questioning whether the actor turned politician is qualified for the role of commander-in-chief.

"Fumbling Fred Thompson's flip flop on the 'Fair Tax' issue shows once again that his presidential campaign is just not ready for prime-time," said DNC spokeswoman Amaya Smith. "Next time Thompson should make sure that he's on the same script as his advisors before changing his position on an issue."

The disputed Thompson incident occurred when Doug Ripley, the controller of FairTax.org, asked him about the tax overhaul measure as the former senator greeted supporters at an airport hangar in Houston on July 24.

You can watch the i-CAUGHT video, which was shot by FairTax.org media producer Paul Yeager, by clicking here.

Proposal Repeals Current Tax Code

The federal taxes that would be swept away by the "Fair Tax" legislation include all federal personal and corporate income taxes, gift, estate, capital gains, alternative minimum, Social Security, Medicare and self-employment taxes. In place of those taxes, the "Fair Tax" legislation would impose a 23 percent sales tax.

The legislation would abolish all current federal tax credits and tax deductions. But in order to offset the impact that the new 23 percent sales tax would have on American households languishing in poverty, the legislation would offer a monthly prebate.

The size of a household's prebate would be calculated by multiplying 23 percent -- the size of the new tax -- against the government-established poverty level for a household of that size.

According to the 2007 federal poverty level, an adult with no children would receive an annual prebate of $2,348 (which would come in monthly checks of $196). A married couple with two children would receive an annual prebate of $6,297, which would come in monthly checks of $525.

Five Republican '08ers Have Agreed to Sign "Fair Tax" If Passed

"Fair Tax" supporters opted for a prebate system, which would flow even to the richest Americans, rather than exempting certain staples of life, in the hopes of protecting the new sales tax from the kinds of lobbying-induced loopholes that pervade the current system.

The five Republican presidential candidates who have pledged to sign the "Fair Tax" bill into law are Sen. John McCain of Arizona, former Gov. Tommy Thompson of Wisconsin, Rep. Tom Tancredo of Colorado, Rep. Duncan Hunter of California and former Gov. Mike Huckabee of Arkansas.

So many Republican candidates have agreed to sign the "Fair Tax" bill because the group that is urging its passage, FairTax.org, has proved adept at using its supporters -- which total a quarter of a million -- to birddog presidential candidates.

"We try to keep constant pressure on them," David Polyansky, FairTax.org's chief operating officer, told ABC News.

Giuliani and Romney Resist

Despite FairTax.org's success with five Republican presidential candidates, it has not been able to secure commitments from either the party's national front-runner or the candidate leading in the crucial states of Iowa and New Hampshire.

The Republican presidential candidate who has been most outspoken in expressing objections to the fair tax proposal is former New York Mayor Rudy Giuliani, the front-runner in national polls.

Asked if he would sign the "Fair Tax" bill, Giuliani said, "I don't think so. I don't think so. I'll have to study it some more. I don't think a fair tax is realistic change for America. Our economy is so dependent upon the way our tax system is operated the best thing to do is to simplify that tax system."

Watch Giuliani's response here.

A second top Republican -- former Massachusetts Gov. Mitt Romney -- has also refrained from pledging to sign the "Fair Tax" bill.

"That's too much of a hypothetical, since you'd have to know the specifics of the legislation," Romney spokesman Kevin Madden told ABC News. "The governor has said that the fair tax idea has attractive characteristics, but it is not part of his tax reform platform. Romney believes in a simpler, less burdensome tax system. He wants to reduce the tax burden placed on Americans, encourage economic growth and simplify the tax system."

"'Fair Tax" supporters have also not been successful in making inroads among Democratic presidential candidates. Among the Democratic White House hopefuls, only the longest of long shots -- former Alaska Sen. Mike Gravel -- has agreed to sign the fair tax bill if passed by Congress.

Feasibility Questioned

Even though the fair tax is marketed as "revenue neutral," liberal tax policy experts have substantial questions as to whether a 23 percent retail sales tax on its own would adequately cover the current cost of government.

"Even before looking at the distributional analysis, there is this big problem that it would leave the basic job of government undone because it would not raise the revenue," said Aviva Aron-Dine, a policy analyst with the liberal Center on Budget and Policy Priorities.

"That said," continued Aron-Dine, if you did set the tax rate high enough to cover the current cost of government, "you would be shifting the tax burden from people at the top of the income scale to people in the middle."

ABC News' Leigh Hartman contributed to this report.