Update: And the Romney people respond with deep voodoo, invoking the supposed fabulous growth effects from his tax cuts. And who could argue? Remember how the economy tanked after Clinton raised taxes? Remember how great things were after Bush cut them? Oh, wait.

More seriously, we have lots of empirical work on the effects of tax changes at the top — and none of it supports the Romney camp’s claims. What we’ve just learned is that they were faking it all along. There is no plan to offset the tax cuts; Romney is just intending to blow up the deficit to lavish favors on the wealthy, then use it as an excuse to savage Social Security and Medicare.

Lots of justified talk today about a new Tax Policy Center report on the distributional implications of the Romney tax plan, showing it to be very much a Dooh Nibor – that is, reverse Robin Hood – thing. Obama is talking it up; Romney, predictably, is dismissing the report as the work of a “liberal group”.

The question one might ask is, did TPC – which is actually painstakingly and painfully nonpartisan – make questionable assumptions to get its results, so that some other set of assumptions might portray Romneynomics in a more favorable light? And the answer is no: TPC actually bent over backwards to literally give Romney every possible benefit of the doubt.

Here’s what they did. They took Romney at his word that he plans to offset his cuts in income tax rates by broadening the base, that is, limiting exemptions and other loopholes. They also assumed, however, that Romney would not be willing to tax dividends and capital gains as ordinary income, since he has made it clear that he opposes any rise in taxes on investment income. As they point out, this leaves a relatively small pool of loopholes to close – big enough that the Romney tax cuts could, in principle, be paid for by base broadening, but not with a lot of room to spare.

So which loopholes are closed? TPC made the most Romney-friendly assumption they could – namely, that base broadening is concentrated on top incomes as much as possible. First you eliminate all deductions that benefit those with more than $1 million in income; then all that benefit those with between $500,000 and $1 million; and so on.

The key point is then that even if you do this, the tax cuts Romney gives high-income Americans are bigger than the loopholes he could conceivably close:

This means that even on the most favorable assumption, the Romney plan would give the rich big tax cuts on net – which means that to be revenue-neutral, it must raise taxes on Americans making less than $200,000 a year.

So they’re actually giving Romney every possible benefit of the doubt – and still his plan is a redistribution from the middle class to the rich. In practice it would surely be much worse.

I’ll be curious to see how all this gets reported. The TPC results are ironclad; there is no valid Romney counterargument except to say that he doesn’t really mean all that stuff about actually making up for lost revenue. My guess is that the stories will nonetheless be he said/she said, but maybe I’ll be favorably surprised.