Doug Elmendorf (2009, slide 11), director of the CBO, provides some sobering calculations. The report’s Alternative Fiscal Scenario estimated that if federal revenues remained in the neighborhood of 20 percent of GDP for the next 75 years, while federal expenditures net of interest rose to 35 percent of GDP (roughly similar to the 2011 CBO estimate), adding interest payments from the accumulating national debt would drive total federal expenditures up to 75 percent of GDP by 2083, and that would entail interest amounting to a fantastic 40 percent of GDP. Obviously government finances will have reached an explosive tipping point long before that level is ever reached.