From an interview with Ezra Klein:
"When Republicans say the first thing you do when you do deficit reduction is reduce rates, it would be like Democrats saying the first thing you do when you do deficit reduction is provide free Medicare at age 55. We’d like to do that! But it won’t bring the deficit down. That’s for sure.
"It just makes no sense and I’m surprised so many have swallowed it for so long. If your number one goal is deficit reduction, you don’t start out by lowering the rates. You don’t need a PhD in economics to understand that."
UPDATE: Okay, it's true that lowering the rates may lessen the magnitude both of the existing distortions in the tax system, and of political incentives to create more loopholes via exclusion or deferral. But as an economic matter (contrary to the mantra of "broaden the base, lower the rates") a broader base and a higher rate are complements, not substitutes.