My colleague Greg Sargent insists that by Mitt Romney saying “The president’s ad saying I’m gonna raise taxes on the middle class? That’s patently, simply false,” Romney is admitting his plan won’t be revenue neutral. Not only does Romney not say that, but that very broad proposition is not supported by the left-leaning Tax Policy Center study released last week.

Like the Simpson-Bowles plan, Romney’s plan wants to lower individual tax rates. To do that he will broaden the base (take away all or part of the personal exemption and/or deductions that reduce the income on which you pay taxes, i.e., “taxable income”). Simpson-Bowles did this same exercise and maintained progressivity just as Romney’s plan would. The difference is that Romney uses all the “savings” from base broadeners to lower the tax rates for all taxpayers while Simpson-Bowles used part of that to reduce the debt.

The lower rates are “paid” for in Romney’s proposal by eliminating deductions. The progressivity is maintained by taking away more of the deductions and exemptions from the rich than from lower-income taxpayers. This, incidentally, was the same concept at play in the 1986 tax reform bill. (More about that below.)

I agree with my colleague Glenn Kessler that this is “a bit like nailing Jello to a wall.” But accusing Romney of only using as much detail as Simpson-Bowles did is not the same as saying the concept can’t work. The exercise is identifying which deductions to target and at what level to phase them out. It also turns on what Romney would include in taxable income.

This is not, by the way, to say that the plan is politically attainable. It is just to say that the very partisan Tax Policy Center didn’t prove, as its media advocates have said, that it is “impossible” for Romney’s plan to be revenue neutral and maintain progressivity. I intuitively understood this because Romney’s plan is conceptual, not written in legislative detail. How would TPC be able to fill in the blanks that Romney left blank? It turned out I was more right than I knew when I began digging. With help from a certified public accountant specializing in high-net-worth individuals and from a conservative tax analyst at an advocacy group (with no connection to the Romney campaign), here is what I found:

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. (Granted, this is not tax simplification, but that is another matter entirely.)

Likewise, TPC states that “we examine only changes to the individual income tax, alternative minimum tax, payroll tax, and estate tax. We ignore the effect of the proposal to reduce the corporate rate to 25 percent.” Why? Romney may want to adjust the ratio of corporate to individual tax revenue.

Elsewhere, TPC says it assumes for base-broadening purposes that municipal bond interest and insurance contract build-ups would not be included in taxable income. But why not? If you’re going to lower the rate for top income earners these are precisely the sorts of items you would include to broaden the base for those in the top bracket.

There are other problems as well. As I read it, the TPC eliminates the death tax but keeps the step-up-in-basis provision for heirs. You have to choose one or the other. It also treats refundable credits for low-income taxpayers as a tax expenditure rather than looking to the spending side to account for checks going out to non-taxpayers. And TPC gives short shrift to dynamic scoring. (For a comparison in the treatment of dynamic scoring take a look at a 2011 piece by Martin Feldstein.)

In sum, if you assume a bunch of stuff, include a bunch of stuff that Romney never said and then leave out all of Romney’s other policies, then Romney’s plan doesn’t work. TPC does this in the guise of “assumptions,” which President Obama’s ads and pro-Obama pundits leave out. (This is not unlike the Obama team’s “outsourcing” gambit, in which a much more limited factual assertion gets used for a much broader, unsupported political assertion.)

Had Democrats relying on TPC made more limited claims (“It shows how hard it is to take away deductions” or “It says it doesn’t work unless you put in previously nontaxable income for the wealthy”), they would have been on firmer ground. It also would have been correct to say that Romney doesn’t give enough detail to determine whether his plan is possible. That is true, but there is no requirement (other than from critics) that a candidate has to make a campaign proposal scorable.

In short, TPC does not provide the silver bullet for slaying Romney’s tax plans that Obama supporters would tell you it does. All it says is that a left-wing tax think tank can come up with a hypothetical in which Romney’s plan would not be possible.

Romney supporters would also argue that Romney’s corporate rate cuts are pro-growth. That is, they would raise additional revenue. And, of course, if we get past the Obama recession and see more people get jobs, this also would affect both the total tax revenue and the distribution of the tax burden under Romney’s plan. But quite apart from these general policy assertions, TPC’s study doesn’t prove what Obama spinners would like it to.

As a final aside, the standard for specificity in a campaign Romney is supposed to adhere to, if you listen to Obama’s team and/or the media, is a far cry from the way it worked in 2008 or from that applied in this campaign to Obama. In 2008 it was fine for Obama simply to say he wouldn’t raise taxes on the rich and would go line-by-line through the budget (neither of which turned out to be true). So far, other than the Buffett tax gambit and a hike in corporate tax rates, we’ve seen nothing from Obama that comes close to the level of detail provided by Romney’s team on tax reform.