Cain, the former CEO of Godfather's Pizza and the only black candidate on the Republican side, also defended his "9-9-9" tax plan, The Detroit News reported. The plan would scrap the current federal tax code, temporarily imposing flat 9 percent rates on personal and corporate income and a 9 percent national sales tax, leading to a national sales tax with no income taxes.
A long-closed train station was the backdrop for the speech.
Critics say the "9-9-9" plan would actually raise taxes for most U.S. residents, with the poor seeing the largest increases. The Tax Policy Center in Washington estimated the increase for those with incomes between $10,000 and $20,000 at close to 950 percent.
Cain said Friday his plan would not increase taxes on the poor, CNN reported.
"If you are at or below the poverty level, your plan isn't 9-9-9 it is 9-0-9," Cain said. "Say amen y'all. 9-0-9."
The opportunity zone plan would give additional tax breaks to businesses in cities with high unemployment like Detroit.
"One of the things that I believe in is empowering cities to help themselves," he said.
Cain and former Massachusetts Gov. Mitt Romney top the field in polls of Republican voters. But many political analysts believe Cain's surge is temporary, a result of disenchantment with Texas Gov. Rick Perry, the former front-runner, and tepid feelings about Romney.
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