In his State of the Union speech, President Obama’s called for a new law that would require high-income people to pay at least 30 percent of their income in taxes. In response, Senator Sheldon Whitehouse (D-RI) and Representative Tammy Baldwin (D-WI) have introduced the Paying a Fair Share Act of 2012, a proposal designed to meet the Buffett Rule: That the wealthy pay at least as much tax as middle-income households.

That sounds straightforward but it’s not.

First, there’s the matter of how to measure income. The rule would define income as adjusted gross income minus a modified measure of charitable contributions. The adjustment avoids discouraging charitable donations