Donald Marron does the math:

The Congressional Budget Office estimates that tax revenues will rise from 14.9% of GDP in 2010 to 20.7% in 2020 and 23.3% in 2035 if current law remains in place...

That rapid growth reflects six factors. First, the economy will recover, lifting revenues from currently depressed levels. Second, the 2001 and 2003 tax cuts will expire, as will tax cuts enacted in the 2009 stimulus. Third, the Alternative Minimum Tax, which is not indexed for inflation, will boost taxes for millions more taxpayers. Fourth, the new taxes that helped pay for the recent health legislation will go into effect. Fifth, retiring baby boomers will make more taxable withdrawals from tax-deferred retirement accounts. Finally, in a phenomenon known as bracket creep, growing incomes will push taxpayers into higher brackets and reduce their eligibility for various credits. Together, those six factors will increase tax revenues by 8.4 percentage points of GDP over the next 25 years, according to CBO.

And then the deadly kicker:

Our long-run fiscal dilemma exists because the scheduled growth in future spending is even larger than the scheduled growth in future revenues.

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