What philosopher Harvey Mansfield calls “taming the prince” — making executive power compatible with democracy’s abhorrence of arbitrary power — has been a perennial problem of modern politics. It is now more urgent in the United States than at any time since the Founders, having rebelled against George III’s unfettered exercise of “royal prerogative,” stipulated that presidents “shall take care that the laws be faithfully executed.” Serious as are the policy disagreements roiling Washington, none is as important as the structural distortion threatening constitutional equilibrium. Institutional derangement driven by unchecked presidential aggrandizement did not begin with Barack Obama, but his...