As Josh Barro of Bloomberg View explains, Mitt Romney will have a very difficult time using cuts to tax preferences to cut all marginal tax rates by 20 percent while maintaining revenue neutrality and preserving progressivity. The reason is that …:
[t]he 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.
My guess is that if Mitt Romney is elected president, the eventual tax code overhaul will look quite different from his campaign proposal, particularly if Democrats retain a majority in the U.S. Senate.