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Mitt Romney's campaign is hitting back against a study released this week showing the GOP candidate's tax plan would raise taxes on the middle class while slashing the tax burden for millionaires, calling the report "biased" and "a joke."

"My plan is very clear," Romney told reporters after an event in North Las Vegas on Friday. "I will not raise taxes on the American people. I will not raise taxes on middle income Americans."

The study was developed by the centrist Tax Policy Center and authored by economists who have worked under both the Obama and Bush administrations.

It concluded that Romney's proposed tax cuts - which include reducing all personal income taxes rates by 20 percent, eliminating the estate tax and zeroing-out taxes on investment income for couples earning less than $200,000 per year - would slash $360 billion in federal revenues in 2015 and will have to increases taxes on the middle class to pay for those losses.

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Romney senior adviser Eric Fehrnstrom was less diplomatic Thursday, calling the study "a joke" and its authors "biased" because one, Adam Looney, worked for the Obama Administration and other has visited the Obama White House a dozen times.

In order to make up for the tax cuts, Romney said he would "limit deductions and exemptions" although he has not specified which loopholes he would close or how he would amend the tax code so that more people pay income taxes. Only 47 percent of Americans paid income taxes in 2010.

But because Romney's proposed tax cuts are so large, according to the study, he would have to slice 65 percent of all the loopholes and deductions that are feasible to cut, such as the mortgage interest deduction, child credit, deductions for charitable contributions and exclusions for employer-provided health insurance.

"You're kind of not left with very much to work with," said Looney, a senior fellow at the Washington, D.C.-based Tax Policy Center, formed as a nonpartisan venture by the Brookings Institution and the Urban Institute.

"The purpose of that paper was to scale back people's expectations for what could actually be reduced in a tax reform," Looney continued, "to establish what's really feasible and what's on the table."

Romney disagreed with this analysis, telling reporters outside his event in Nevada Friday that if his plan was assessed "properly" it would not "in any way" raise taxes on the middle class.

"I want to limit deductions and exemptions for high income people, but high income people, when you finish going through my plan, and we score it properly, I will not have a plan that lowers the share paid by high income folks or that in any way raises taxes on middle income Americans," Romney said.

Under Romney's plan, the Tax Policy Center estimates that high-income earners would get the largest tax cut. A family that makes between $200,000 and $500,000 would pay about 6 percent less per year under his plan while families earning between $50,000 and $75,000 would get a 2 percent cut.

The cuts for high-income earners are so large that eliminating every feasible tax deduction and loophole and exemption for people making more than $200,000 would not pay for tax cuts they would receive, the study finds. So among people who earn more than $200,000 per year, it would not be possible to pay for Romney's proposed tax cuts by reducing the exemptions and deductions that group receives.

"Turns out there are not enough of those tax breaks that benefit high income taxpayers to make it rev neutral just within that group," Looney said. "That's not a judgment, it's a mathematical fact of life."

So in order to make sure his plan would not add to the deficit, Romney would have to pay for $86 billion worth of high-income tax cuts by cutting deductions that benefit middle- and low-income earners, according the Tax Policy Center study.

If his plan is to be revenue neutral, as Romney has said it would be, the study shows he would have to raise taxes on 95 percent of Americans - families earning less than $200,000 - by an average of $500 per year. Millionaires will still get an $87,000 tax cut.

"They [tax plans like Romney's] seem to be discussed in a way that I'm going to have really big cuts in tax rates, but the tax system is going to be just as progressive as it was before, that the impact on tax revenue or on the budget is going to be the same and I'm going to maintain all of your treasured deductions," Looney said. "At some point, the math just doesn't add up.

"That's what we were aiming to show," he added.

In order to avoid such middle-class tax hikes, Looney said, Romney has three options: change the plan he has already laid out so it does not give such large tax breaks to the wealthy, drop the revenue-neutral requirement so that the tax cuts are not offset with revenue increases but instead add to the deficit, or make up for the lost revenue from his tax cuts by slashing spending.

Romney's campaign has argued that his plan to cut tax rates would spur economic growth. And as the economy grows, allowing more people pay more taxes, his plan would be virtually paid for by those increased revenues.

Looney said the Tax Policy Center ran its study again using the economic-growth scenario laid out by Romney's economic advisers that estimates his plan would add 12 million jobs and increase GDP growth by 1 percent, which Looney said was "an implausibly large estimate."

Romney's proposed economic growth would offset 15 percent of the tax cuts, but would still leave the GOP candidate with $307 billion less in revenues.

"Even in that case, there's still a shift in the tax burden from high-income taxpayers to low -and or middle-income taxpayers," Looney said. "It's smaller, but it would require a net tax increase on the middle class."