Some policymakers and pundits have argued that currency manipulation–the effort to keep exports cheap by intervening in the foreign exchange market–is the most important issue international economic policy could address. Friday evening, the Senate rejected an amendment by Sens. Rob Portman (R., Ohio) and Debbie Stabenow (D., Mich.) that would have punished currency manipulators. The White House has responded to pressure from Congress, signaling a willingness to bring exchange-rate provisions into trade negotiations, but does not want binding sanctions against currency manipulators to be part of the Pacific trade agreement under consideration. Currency manipulation is not like pornography–you don’t know it when you think you see it. It’s hard to define and even…