Today's Washington Post takes Senator John McCain to task for promising unsustainable tax cuts, including making permanent President Bush's tax cuts, repealing the AMT, making permanent the research and experimentation tax credit, cutting the top corporate tax rate from 35 to 25 percent, and allowing businesses to immediately write off all capital investments (rather than depreciating them over time). He has also proposed to require a super-majority vote in Congress for any tax increases. (Last week, TaxVox raised its own questions about how tough it would be for McCain to keep his tax cut promises.)

Some conservatives back huge tax cuts because they think the government would just waste the money that would otherwise pour into government coffers. While it's true that a growing economy (and the voracious AMT) would cause federal revenues to grow to over 20 percent of GDP by 2018 under current law, the Post reports (based on my estimates) that Mr. McCain's tax cuts would slash tax revenues to 16.6 percent of GDP in 2018. By comparison, the postwar average of tax revenues was over 18 percent of GDP, and the federal government will face enormous pressures as the baby boomers begin to retire.

Here is a table showing my calculations.

Note that the 16.6 percent estimate understates the restraints that the McCain proposal would put on federal finances. If you subtract the additional interest that would have to be paid on the national debt (based on CBO interest rate projections), the proposed tax cuts would reduce federal revenues available for non-interest expenses by 5 percent of GDP.

To put these numbers in context, federal tax revenues were 16.5 percent in 1955. For most of the Eisenhower administration, they were significantly higher. If Mr. McCain is prepared to eliminate some of the big-ticket items enacted since then, especially Medicare and Medicaid, his plan might work. Otherwise, it is a recipe for fiscal disaster.

Here are the sources for my calculations: CBO Budget and Economic Outlook, 2008-2018, US Treasury Blue Book, FY2009, Tax Policy Center (table t08-0043, extrapolated to 2018). The estimates assume that expensing of corporate investment and rate reduction would be effective October, 2009, and reduce corporate revenues by half (which is probably a conservative estimate).