President Obama has let Elizabeth Warren veto presidential appointments, and the power rush seems to have gone to her head. Now the Massachusetts Senator has forced the resignation of a Brookings Institution economist because he dared to report that new financial regulations will cost investors.

Robert Litan, a Democrat who has been affiliated with Brookings for decades, is nobody’s idea of a conservative. And he’s not philosophically opposed to financial regulation. He was among the first to endorse Ms. Warren’s proposal...