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.
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.
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.