The former New York City mayor cited the struggling U.S. housing market as a reason to avoid the plan, which would eliminate all taxes on income and investments in favor of a hefty federal sales tax.
"I think there are several tax deductions that are vital to our economy," Giuliani said. "This would not be a good time - I don't know if there would ever be a good time to do this - to advocate ending the home mortgage deduction. The home mortgage deduction is considered by many critical to the ability of people to buy a home and keep their home."
He also said deductions for charitable contributions and state and local taxes were important tax breaks that Huckabee's plan would eliminate. The plan calls for getting rid of the Internal Revenue Service and giving taxpayers a monthly rebate on taxes on purchases up to the poverty line.
"Why waste our time trying to do this if we can't actually accomplish it?" Giuliani said. "I try to put my time into things that we can actually accomplish."
Still, Giuliani said he, too, wants to simplify the tax code so a return could be completed with one piece of paper.
Huckabee is running about even in Iowa with Mitt Romney, according to a Sunday poll in The Des Moines Register. Huckabee is at 29 percent to Romney's 24 percent with a 4.4 percentage point margin of error. Giuliani is at 13 percent.
Giuliani spent much of his time in Greensboro criticizing Democratic proposals on health care, education and pensions. He said the Democratic presidential candidates touting those programs are pushing the country toward more taxes when it needs more investment in private business.
On education, he discussed his plan for school vouchers, saying a lack of choice in education "may be the biggest civil rights issue" of the day. Giuliani said he - not the Democrats - was best suited to handle the millions of Americans who live in poverty.
"We have to have a society that rewards success," Giuliani said. "The only way in which we get people out of poverty is you help them help themselves by giving them the opportunity for a good education and the opportunity for a good job. That's how you deal with poverty - not with government welfare programs."