In other words, even the president’s most core powers may not be abused for wrongful purposes—and a president firing an FBI director to protect himself from an espionage investigation would seem about as wrongful as it gets.

But there’s more to be said than that.

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.

The ancestor of the FBI was created in 1908, during the administration of Theodore Roosevelt. From the start, critics of the new agency fretted it might act as a political police force, beholden to the president—and those fears were rapidly proven right. In an effort to beat back the Teapot Dome scandal, Attorney General Harry Daugherty prevailed upon the Bureau of Investigation to unearth dirt on the Harding administration’s opponents in Congress. Failing to find anything salacious enough for Daugherty, the Bureau collaborated with the attorney general actually to manufacture false accusations. The ensuing scandal toppled both Daugherty and then Director William Burns—and indelibly stained the reputation of President Harding as well.

The new Coolidge administration appointed J. Edgar Hoover director in 1924, with a mandate to professionalize the bureau and safeguard its political independence. Hoover’s FBI would generate plenty of abuses of its own—but whatever its other faults, it emphatically refused to accept Dershowitz’s recommended theory of presidential power. Richard Nixon, who hated and feared Hoover, dared not fire him—and even reappointed him to a final term in 1971.

And when Hoover’s death at last provided Nixon the opportunity to replace Hoover with a trusted supporter, L. Patrick Gray, that very association doomed Gray’s hopes of Senate confirmation. Gray served as acting director for only 11 months. He was forced to resign after it was discovered that he had shredded Watergate-relevant documents at the White House’s behest. Gray’s resignation in April 1973 crystallized a new set of norms about the FBI.