President-elect Donald Trump’s goal of overhauling the U.S. tax code in 2017 will depend partly on the work of an obscure congressional committee tasked with estimating how much future economic growth will result from tax cuts. Known as the Joint Committee on Taxation, or JCT, the nonpartisan panel assigns “dynamic scores” to major tax bills in Congress, based on economic models, to forecast a bill’s ultimate impact on the federal budget. The higher a tax bill’s dynamic score, the more likely it is seen as spurring growth, raising tax revenues and keeping the federal deficit in check. As Trump and...