It’s disappointing that Martin Feldstein, once the champion of Reaganomics, would now need to defend the House GOP’s border-adjustment scheme by arguing that it is necessary for “cutting the corporate tax rate—and stimulating economic growth—without a major increase in the budget deficit.” Three decades ago Mr. Feldstein would have argued that cutting the U.S. corporate tax rate from the highest in the world would decrease the budget deficit precisely because it will stimulate growth.