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Stung by criticism that his signature "9-9-9" economic plan would saddle America's poorest citizens with a disproportionate share of the country's tax burden, GOP presidential candidate Herman Cain announced some new features to the plan, even as he insisted they have been included in his campaign literature all along.

Speaking at a sparsely attended outdoor event, Cain stood before the stately Michigan Central Station in downtown Detroit, where the unemployment levels are among the highest in the country, to announce his proposal to create "opportunity zones" in America's inner cities. His announcement came just days after the nonpartisan Tax Policy Center in Washington released a study of the "9-9-9" plan that concluded the plan would increase taxes by more than 900 percent on U.S. households earning between $10,000 and $20,000 annually.

Accusing his rivals and critics of having never read his plan all the way through, Cain then used language about the plan never heard before - and sure to raise questions about whether his economic proposals now lack the simplicity that once appeared their central virtue.

"If you're at or below the poverty level, your plan isn't ‘9-9-9,'" Cain said, with economic adviser Rich Lowrie and other supporters flanking him. "It's ‘9-zero-9.' Say ‘Amen,' y'all! In other words, if you are at or below the poverty level based upon family size, because there's a different number for each one, then you don't pay that middle ‘9' tax on your income. This is how we help the poor."

A day earlier, Fox News had exclusively reported that Cain's opportunity zone proposal is stirring displeasure among leaders of organized labor. Although he did not dwell on this during his appearance in Detroit, Cain insists that those areas that wish to qualify as an opportunity zone must eliminate what the campaign calls "barriers" to economic growth.

Examples of how to do that, as provided by campaign sources, include a number of steps considered anathema to Big Labor. They include the abolition of the minimum wage; the institution of school choice for parents; and the establishment of "right to work" conditions, which allow workers to refuse to join unions in unionized workplaces.

"It's tough to take anything like that seriously," AFL-CIO President richard Trumka told Fox News. "Look, workers are working hard and their wages have stagnated. To have Herman Cain, a serious contender on the Republican side, make a statement like that - that he wants to further lower wages, he wants to do away with the minimum wage - it's almost laughable."

"Herman Cain's 'opportunity zones' appear to be an opportunity for corporate America to exploit workers and turn the United States into a third-world country," said Teamsters President James Hoffa, in an email to Fox News.