The WSJ reports:
Even if all Bush tax cuts are extended and the AMT is patched, tax revenues will rebound to 18.2% of GDP by 2020—slightly above the historical average. They will continue growing afterwards…
CBO figures show spending…which has averaged 20.3% of GDP over the past 50 years… surging to a peacetime record 26.5% of GDP by 2020 and also rising steeply thereafter.
Putting this together, the budget deficit, historically 2.3% of GDP, is projected to leap to 8.3% of GDP by 2020 under current policies. This will result from Washington taxing at 0.2% of GDP above the historical average but spending 6.2% above its historical average.
Link via Goodman.