Herman Cain will try to clarify his 9-9-9 economic plan during a speech Friday in Detroit. 

The presidential contender is also expected to offer details about the economic "opportunity zones" he has cited as offsetting increased taxes on the poor that could result from his proposal to replace the current federal tax scheme with 9 percent income, sales and corporate taxes. 

Cain has vaulted to the top of GOP voter polls in the contest for the Republican nomination, but is now coming under intense scrutiny for his tax plan, with many conservatives saying it could open up a new federal sales tax that could lead to higher taxes. 

Other groups have said the plan itself would raise taxes on the poor and middle class while giving the rich a huge tax break. 

Cain has said these are misreadings of his proposal, but the focus on "opportunity zones" appears to be a response to at least some of this criticism. 

Cain hopes to encourage growth in impoverished areas by further lowering the tax burden of residents. But for a jurisdiction to qualify, it would have to adopt a number of conservative policies that are unpalatable to liberals, including eliminating the minimum wage, instituting school vouchers and declaring the area "right-to-work" — or non-union.

A preview of the speech was obtained in an email mistakenly sent to Fox News.

"As part of his bold vision for the American economy, Presidential candidate Herman Cain unveiled his Opportunity Zone Plan, designed to create an environment in designated communities that allows production to drive the economy, not spending; which encourages risk-taking to drive growth, speaking Friday outside of Michigan Central Station in Detroit, Michigan," the email read.

It appeared from the tense of the email that the campaign intended to distribute the release after Cain spoke. It went on to quote from Cain's remarks, which will be delivered later on Friday:

"Addressing recent questions about the "9-9-9 Plan," Cain said, "we carved out a substantial amount from the aggregate "9-9-9 Plan" tax base, enough to exempt those in poverty, and we will work with Congress to best apply these in a way to break the "poverty trap" and replace it with positive incentives that encourage people to work and take risks in this economy."

An adviser to Cain also told Fox that jurisdictions could apply for waivers in areas where city or state law might prevent them from adopting the empowerment zone requirements.

Cain's plan came under fire earlier this week when the nonpartisan Tax Policy Center reported that 84 percent of U.S. households would see their taxes rise, with tax burden shifting dramatically from the most wealthy to the poor. According to the center, Cain "raises taxes on typical middle-income households by more than $4,000 while cutting them on those with the highest incomes by an average of $1.4 million."

Cain has argued that the analysis does not incorporate the implementation of these empowerment zones. But, as the Tax Policy Center notes, "because his plan is roughly revenue neutral now, [tax empowerment zones] would force him to either increase the deficit or abandon that nice sounding 9-9-9 and raise his proposed tax rates."