"Our results indicate that tax changes have very large effects on output.  Our baseline specification implies that an exogenous tax increase of one percent of GDP lowers real GDP by almost three percent.  Our many robustness checks for the most part point to a slightly smaller decline, but one that is still typically over 2.5 percent."  Christina and David Romer, American Economic Review 100, June 2010. (Christina Romer was Obama's Chair of the Council of Economic Advisers until yesterday.)

Source:  Christina and David Romer, American Economic Review, June 2010.