The bigger challenge is crafting a reform that does not hurt the middle class at the expense of the rich. Mr Romney has pledged not to raise the preferential rate on capital gains and dividends; 75% of those benefits go to the top 1% of households. That group also prospers most from Mr Romney’s proposed cut in the top income-tax rate, to 28% from 35%. So Mr Romney would probably have to target the rest of the upper quintile, the upper-middle class who gain from other big tax breaks for employer-provided health care, mortgage interest, retirement savings, state and local taxes and municipal bond interest.

Even if Mr Romney could devise a workable plan, overcoming political resistance is another matter. Optimists note that a 33-year-old tax credit for ethanol, after surviving numerous assassination attempts, finally died last year. But the $6 billion saved by that was trivial. By contrast, Congress has steadfastly ignored Mr Obama’s proposal in his first budget (repeated ever since) to limit the tax break for mortgage interest and charitable deductions for rich households. Both ideas were fiercely condemned by interest groups. “As much as I’m a card-carrying member of the broaden-the-base, lower-the-rates, go-after-tax-expenditures club,” says Mr Marron of the TPC, “the politics and underlying economics suggest it’s hard to raise the money you need for these grand plans.”