<P> CBO estimated under the baseline scenario that the U.S. debt held by the public would increase approximately $8.5 trillion between the end of 2014 and 2024 . Under a $2 trillion deficit reduction scenario during that first decade, federal debt held by the public in 2039 would stand at 75 percent of GDP, only slightly above the value of 72 percent at the end of 2013 . Under a $4 trillion deficit reduction scenario for that decade, federal debt held by the public would fall to 42 percent of GDP in 2039 . By comparison, such debt was 35 percent of GDP in 2007 and has averaged 39 percent of GDP during the past 40 years . </P> <P> CBO reported in September 2011: "The nation cannot continue to sustain the spending programs and policies of the past with the tax revenues it has been accustomed to paying . Citizens will either have to pay more for their government, accept less in government services and benefits, or both ." </P> <P> In January 2017, the Congressional Budget Office reported its baseline budget projections for the 2017 - 2027 time periods, based on laws in place as of the end of the Obama administration . CBO forecasted that "debt held by the public" would increase from $14.2 trillion in 2016 to $24.9 trillion by 2027, an increase of $10.7 trillion . These increases are primarily driven by an aging population, which impacts the costs of Social Security and Medicare, along with interest on the debt . As President Trump introduces his budgetary policies, the impact can be measured against this baseline . </P> <P> CBO also estimated that if policies in place as of the end of the Obama administration continued over the following decade, real GDP would grow at approximately 2% per year, the unemployment rate would remain around 5%, inflation would remain around 2%, and interest rates would rise moderately . President Trump's economic policies can also be measured against this baseline . </P>

Where does the largest source of federal revenue come from