From post by Dylan Matthews on The Washington Post's website:
The Joint Committee on Taxation is one of the most respected institutions in Washington. It is to the tax code what the CBO is to the budget: a nonpartisan, impartial arbiter that produces respected estimates of revenues and other figures important to tax policy.
So it’s a big deal that they sent Sens. Max Baucus and Orrin Hatch -- the chair and ranking member, respectively, on the Senate Finance Committee -- a letter (obtained first by Bloomberg) arguing that even if you eliminated almost every tax deduction in the books, you could only afford to cut tax rates by 4 percent before you start adding to the deficit.
(The post contains the full text of the report to Senators Baucus and Hatch.)
Contrast this with Mitt Romney's claim that it's possible to cut all personal income tax rates by 20%, eliminate the estate tax and alternative minimum tax, and still not increase the deficit, simply by limiting some unspecified deductions used by high-income people.
There's a longer article about this on the Bloomberg Businessweek website, which broke the story Saturday.