Every time control of the U.S. presidency is passed from one party to another, the entire top layer of the executive branch changes. Thousands of men and women take down their pictures, pack up their desks, and move back into private life, just as others dust off their pictures and move in. The U.S. stands alone in this respect. Nearly every other advanced democracy is managed-save for elected officials and a few top aides-by an elite cadre of top...