At least some Democrats are resisting Sen. Elizabeth Warren’s purge of the liberal intelligentsia. This week Ms. Warren succeeded in forcing the resignation of respected scholar Robert Litan from the Brookings Institution after he revealed that a new Labor Department regulation could cost investors billions. Now five Democratic economists have authored a letter to protest Warren’s bullying. Robert Lawrence of Harvard’s Kennedy School and Bowman Cutter of the Roosevelt Institute are among those writing “to express our concern over our colleague Bob Litan’s treatment at the hands of the Brookings Institution and Senator...