Tax revenue as a percentage of GDP peaked in 2000, and has since declined from its relative maximum (21%) to around 14.9% in 2010. Due to the extension of the Bush tax cuts, this ratio is projected to fall to 14.5% in 2011 before picking up in 2012.
Which brings me to automatic stabilizers, e.g. unemployment insurance, food stamps, medicaid etc....
Research from the Federal Reserve Bank:
If you are looking for something to blame for deficit spending, blame the recession.This paper provides quantitative estimates of the effects of the automatic stabilizers on
the government budget and on the economy. We find that at the general government level each
1 percent increase in the GDP gap increases the deficit by 0.45 percent of GDP with 0.35 percent
of GDP occurring at the federal level. According to simulations with FRB/US, the automatic
stabilizers provide a moderate amount of buffering of aggregate demand shocks. The stabilizers
attenuate the effects on aggregate demand by about 10 percent after four quarters and 20 percent
after eight quarters. Turning to active fiscal policy, the federal government has engaged in
countercyclical policies following most business cycle peaks. This has been offset to some
degree by tightening at the state and local level. During 2008-09, the combined effects of federal
and state and local budgets on aggregate demand (from both discretionary actions and automatic
stabilizers) may have lifted the level of GDP by 2½ percent in 2009.