Back in the 1980s, many commentators ridiculed as voodoo economics the extreme supply-side view that across-the-board cuts in income-tax rates might raise overall tax revenues. Now we have the extreme demand-side view that the so-called "multiplier" effect of government spending on economic output is greater than one -- Team Obama is reportedly using a number around 1.5.

To think about what this means, first assume that the multiplier was 1.0. In this case, an increase by one unit in government purchases and, thereby, in the...