Conservatives have reacted aggressively against the TPC report. It seems that Mitt’s plan should be viable: If you cut tax rates proportionally across the board, and eliminate tax deductions proportionally, it seems progressivity should be unchanged. In fact, if you eliminate tax breaks starting with the wealthy, as Romney says he would, it seems he should be able to make the tax code even more progressive.

The idea is intuitive, but wrong. And it’s wrong because of something people don’t realize: The tax preferences that exist today overwhelmingly benefit people with lower and middle incomes, not the wealthy. While tax rate cuts reduce income tax burdens proportionally, as TPC notes, there aren't enough tax preferences for wealthy people to offset Romney's cuts at the top.

To understand this, we can look at the IRS Statistics of Income report for 2009, the most recent year available. Tax returns reporting less than $200,000 of adjusted gross income (AGI) accounted for a total AGI of $5.86 trillion, and taxable income of $3.24 trillion. That is, deductions and exemptions amounted to 45 percent of adjusted gross income for people making under $200,000.

Tax returns with more than $200,000 of AGI (the highest-earning 2.8 percent of filers) had a total of $1.96 trillion in AGI and $1.62 trillion in taxable income. For this high-income group, deductions and exemptions were just 18 percent of adjusted gross income