Interest on the debt, currently it's about 200 Billion (10% of tax receipts). In 2020, it will be 800 Billion (40% of the federal budget), if you add in medicare and SS then boom you're over 100%. Exponential growth while slow at the start, can relatively quickly become quite large.

http://investmentwatchblog.com/interest-expense-on-the-us-de...