Congress has a dreadful record predicting the economic impact of its policies, in part because it relies on computer models that are as reliable as tarot cards. So it's a good sign that before going on recess a majority of the Senate endorsed "dynamic scoring" of changes in tax law.

For decades the official forecasters at the Congressional Budget Office and Joint Committee on Taxation have assumed that changes in tax rates have little impact on how businesses and households behave or on the competitiveness of the...