Loren Adler and Shai Akabas explain:
Even if the 2001, 2003, and 2010 tax cuts are extended … revenues are projected to rise from 16.1 percent of GDP in 2012 to 18.0 percent in 2017, and then remain roughly at that level for the remainder of the decade. This approximates the average U.S. revenue level in the modern era. In times of better fiscal health, such receipts might have been sufficient, but the coming decades do not fit the bill.
First, our public debt is already at 72 percent of GDP – higher than it has been in over half a century – and projected to increase further. Second, a wave of baby boomers are beginning to descend on the entitlement programs, and supporting them in retirement will require additional spending even if significant reforms are made to control costs. Between these two trends, we will not be able to collect revenues at "normal" levels if we want to have a sustainable budget and a functioning economy.