President Obama today endorsed a “Buffett Rule” for tax policy: “Any reform should follow another simple principle: Middle-class families shouldn’t pay higher taxes than millionaires and billionaires. That’s pretty straightforward. It’s hard to argue against that. Warren Buffett’s secretary shouldn’t pay a higher tax rate than Warren Buffett. There is no justification for it.” Obama is arguing for another “alternative minimum tax (AMT).” In this case, the “Buffett Rule” seems to imply that high income households should pay taxes (even on capital gains) at about a 25 percent rate, no matter what. That is the marginal tax rate for most...