TPC claims that Romney cannot cut tax rates by 20 percent across the board and maintain revenue neutrality without raising the net tax burden on the middle class. According to TPC, in the year 2015 under Romney's tax plan, "maintaining revenue neutrality mathematically necessitates a shift in the tax burden of at least $86 billion away from high-income taxpayers onto lower- and middle-income taxpayers. This is true even under the assumption that the maximum amount of revenue possible is obtained from cutting tax expenditures for high-income households."

So, according to the Tax Policy Center, we start out with an $86 billion hole in Romney's tax plan. But the Tax Policy Center's own calculations show that that $86 billion hole can, in fact, be filled without raising middle class taxes.

TPC's study assumes that pro-growth tax reform cannot produce any economic growth. TPC acknowledges that, according to an economic model created by Harvard professors Greg Mankiw and Matthew Weinzerl that assumes tax reform will produce economic growth, "the tax cuts would result in revenue reductions of $307 billion (instead of $360 billion)." In other words, economic growth could fill $53 billion of that $86 billion hole.

That still leaves us $33 billion short. But TPC also acknowledges that its study assumed that Romney would not touch "the exclusion of interest on state and local bonds and the exclusion of inside-buildup on life insurance vehicles." According to TPC, eliminating these exclusions could raise $45 billion in revenue.

So economic growth ($53 billion) plus nixing these two exclusions ($45 billion) equals $98 billion. That's $12 billion more than the $86 billion needed to prevent a middle class tax hike.

William G. Gale, co-director of the Tax Policy Center and one of the authors of the study on Romney's tax plan, told me this morning that under these two assumptions Romney's tax plan would maintain revenue neutrality without raising middle class taxes. "Under those assumptions and policies it would be revenue neutral," Gale wrote in an email, "but remember the tax expenditures are eliminated from the top down and that is not administratively feasible. So you also have to assume tax expenditures are eliminated in an infeasible manner to avoid the tax increase on households with income below $200,000."

So what the Tax Policy Center is really saying is not that Romney's tax plan is mathematically impossible, but that it's difficult. Whether or not eliminating tax expenditures from the top down is "administratively feasible" is a matter of opinion--not a matter of math.