The Constitution decrees the basic operating rules of American government, but it offers only the haziest description of how that government actually works. Within the Constitution, there have grown a set of practices and habits that Americans today take for granted as integral to a free society. The president nominates the leadership of the Federal Reserve—but he does not dictate monetary policy. The president commands the military, but he should not favor officers known to sympathize with the president’s political party. These rules are no less compulsory for being unwritten—and so it is with the independence of the FBI from the political influence of the president.
If accepted, Dershowitz’s notion that the FBI director ought to be removable by the president at any time for any reason would transform the FBI director into a political appointee like any other—say, the head of NASA. Presidents in the past have indeed from time to time tried to impose that notion. They have, until now, always failed.