IT WAS Greg Mankiw's turn in the New York Times' rotation of weekend economic columnists on Sunday, and Mr Mankiw used his column space to take on the unenviable job of defending John McCain's economic policies. Specifically at issue was Mr McCain's plan to reduce the corporate tax rate from 35% to 25%.

It's an idea with strong support from economists on the right, who note that America's corporate tax rate is high relative to many other nations, and who argue, correctly, that the distortionary impact of the corporate tax is high relative to other means of raising revenue.

But Mr Mankiw makes a number of errors—of omission and commission—in making his case. First and foremost, as Mark Thoma points out, Mr Mankiw suggests that a cut in the corporate tax rate might be self-financing. This is a highly dubious proposition which lacks the support of most academic economists. Following closely from this goof, Brad DeLong sagely notes that while a corporate tax rate cut might be good for economic growth, an unfunded one will not. The long-term budget constraint applies, and while Mr McCain has promised to rein in spending, his actual policy proposals are rather quiet on just where these cuts would come (with the exception of earmarks, which offer too little excess to close the gap).

Finally, there is the matter of progressivity in the tax cut. Mr Mankiw does cite research showing that much of burden of the corporate tax is passed on to labour, and that is no doubt the case. But much of it is not, and as Mr Thoma notes, European nations generally combine their low corporate tax rates with generous redistribution of revenues.

There can be little doubt that a move away from taxation of income and capital, and toward taxation of consumption and negative externalities (like congestion and carbon emissions) would greatly improve the efficiency of the tax code. Politically, that's a difficult case to make. While Mr Mankiw's proposal is likely music to the ears of scholars at the American Enterprise Institute, he'll have to do more to acknowledge its drawbacks if he hopes to win converts from a broader audience.

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