Ceci n’est pas une picnic bench Photo: Melina Mara/The Washington Post via Gety Images

Last week the Tax Policy Center found that Mitt Romney’s tax plan, based on the descriptions he has offered, would have to raise taxes on people earning less than $200,000 a year. Romney has promised a 20 percent rate cut, not to raise taxes on investment income, and to offset the cost from lower rates by broadening the base. Those promises arithmetically add up to higher taxes on the middle class. This is a fundamental reality that Romney, of course, badly needs to deny.

Heroically hurling herself into the breach between Romney’s plan and math comes Jennifer Rubin, the conservative blogger and unofficial Romney spokesperson.

Rubin tries several arguments. She calls the Tax Policy Center “very partisan,” despite having previously endorsed its acumen and nonpartisan standing. She compares Romney’s plan to the proposal endorsed by Bowles-Simpson, which is fundamentally different. (Bowles-Simpson would raises taxes on the rich by ending the capital gains tax break; Romney promises to “further reduce taxes on savings and investment,” which would guarantee the rich would get a tax cut.)

Possibly Rubin’s most interesting and original contribution to the debate is to completely misapprehend TPC’s basic analysis. The method of the study is to grant Romney every possible assumption, assuming that he would design a plan to suck every possible cent out of the wallets of the rich, that the supply-side growth-spurring dreams of his advisors would come true, and of course that political or practical obstacles would disappear. So, on the latter point, the study suggests that trying to fulfill Romney’s irreconcilable promises would be politically difficult, but it ignores the political obstacles.

Here’s how Rubin attempts to process this:

Let’s begin with the “assumptions” made by the TPC. For example, it states that “we have not examined whether it would be politically realistic to reduce tax expenditures — provisions like the mortgage interest deduction, charitable giving, the tax benefit for health insurance, the EITC, and the child tax credit, etc., — by 58 percent for those earning less than $200,000, and to eliminate such features entirely for all households earning more than $200,000.” Why not? That might be precisely what Romney wants to do. TPC is supposed to be telling us if Romney’s plan is mathematically possible, not politically easy to achieve.

Wait. That’s not how logic works. This is a passage in which TPC grants Romney’s assumptions. The authors aren’t saying Romney can’t do something because it’s politically unrealistic. They’re saying that they’ll assume he can do it even though it may not be.

Rubin seems unfamiliar with this form of argumentation. Let me demonstrate the principle. Suppose Rubin were to write, “The liberal media refuse to report that the rightful winner of the Olympic Games is actually Mitt Romney, who defeated the entire U.S. women’s soccer team by himself.” And then I were to reply that, even if Romney had accomplished this goal, which I doubt, he would be ineligible for the gold medal because he is not a woman. It would not rebut my argument for Rubin to insist that Romney had too defeated the U.S. women’s soccer team all by himself. She would have to address the point I am actually contesting — namely, Romney’s gender eligibility to claim the women’s soccer gold medal.

Rubin carries on in the belief that TPC has created its own assumptions about Romney’s proposal, when the entire point of the paper is that it operated only under Romney’s own assumptions. That’s the key finding — if you carry out Romney’s promises to cut tax rates by 20 percent and not increase investment taxes, there are not enough tax deductions available for the rich to offset the cost of the rate cuts. The paper even works up a nifty bar graph showing this. The blue bar is the cost of the rate cuts, and the red bar is all the available savings from tax deductions. Notice how, starting at $200,000 a year, the blue bar is higher than the red bar.

On the Distributional Effects of Base-Broadening Income Tax Reform Photo: Samuel Brown, Bill Gale, Adam Looney

It’s math. The math is so incontestable here that the only rebuttal left is Rubin’s I-don’t-understand-what-words-mean interpretive method.