President Barack Obama's relentless advocacy of the Buffett Rule—legislation to compel Americans earning $1 million or more annually to pay 30% of it in income taxes—moved last Saturday from misleading to incoherent. The occasion was Mr. Obama's weekly radio address, when he explained the tax was "about growth . . . about being able to make the investments we need to strengthen our economy and create jobs."

This was at least the president's third rationale for the new tax. Last September, Mr. Obama told a California fundraiser...