To recap: Romney has proposed a 20 percent across the board income tax cut, to cut corporate taxes, and repeal the Alternative Minimum Tax, among other things. He claims that he will make up the lost tax revenue by closing unspecified loopholes in the tax code. This is where the $5 trillion dispute comes from about Romney's tax plan—his tax cuts are projected to cost around $5 trillion. He argues that it's not fair to characterize his proposal as a $5 trillion tax cut because—you'll have to take his word on this—he's going to offset it by closing loopholes.
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Romney's math doesn't work. Tax loopholes have become the modern equivalent of wasteful spending--a generic and vastly overestimated pool of money politicians can cite as offsets for their expensive policies. The Congress's nonpartisan Joint Committee on Taxation found that if you repealed all itemized deductions from the tax code (as in goodbye mortgage interest deduction), it would only pay for a 4 percent cut in tax rates.

And more specifically to Romney's plan, the nonpartisan Tax Policy Center (whose findings the Romney campaign used to tout), has run the numbers and figured out that the wealthy don't currently get enough breaks in the tax code to pay for the Romney tax cuts. In order to pay for the cuts middle class taxpayers would have to lose expenditures—more than offsetting the tax breaks they would see.

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And while Romney and Ryan have talked about a half-dozen independent "studies" which defend his tax plan, they are actually not studies at all—rather they're three blog posts, an op-ed, and a couple of white papers, one of which was written by Romney's own economic advisers. Oh, and they don't actually back up his plan, according to The Atlantic's Matthew O'Brien.

So understand that while Romney's goal sounds good, it's straight out of campaign fantasy land.