Bloomberg has the simplest, most straight forward break down of why Mitt Romney's tax plan won't work...
Mitt Romney’s tax plan has three key planks. He cuts personal income tax rates by 20 percent across the board; he eliminates deductions, exclusions and credits so that the deficit does not grow; and he doesn’t make the tax code any less progressive. Unfortunately, as the Tax Policy Center has shown, only two of these planks can co-exist.Read the whole thing for the details (you've already read almost half of the original post if you've gotten this far, why stop now).
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.