Mitt Romney showed a little leg on the question of his tax returns today, responding to reporters by saying that he has never paid less than 13% in taxes.

I don’t have a huge problem with the Buffett rule, which will get a vote in the Senate and fail on Monday. But I do have a problem with the concept of a minimum tax based on adjusted gross income as a killer app in tax policy, rather than just raising rates and closing loopholes. To wit, high-income earners already have their tax avoidance strategies lined up.