Fair of Yale University estimates that without big changes in policy, the national debt will rise to 75% of GDP by 2020 from 50% now, while interest expense will rise to 4.3% of GDP (see chart, below).

FORBES: Magazine Article

But whatever we do with tax policy (and also energy and regulatory policy) to increase the GDP, it will not be nearly enough to stop the escalation of our national debt.

FORBES: Tax Rates, Tax Revenues And The GDP