It’s not a constitutional crisis. Technically, President Donald Trump was within his constitutional rights Tuesday when he fired FBI Director James Comey. The Federal Bureau of Investigation is part of the executive branch, not an independent agency. But the firing did violate a powerful unwritten norm: that the director serves a 10-year, nonrenewable term and is fired only for good cause.


Only one director has ever been removed from office involuntarily: President Bill Clinton fired Director William Sessions in 1993 after an internal report found that he had committed significant ethics violations.


There is, therefore, reason to be deeply concerned about Comey’s firing, which has the effect of politicizing law enforcement — a risky precedent in a rule-of-law democracy. And the fact that the FBI is investigating the Trump administration makes that politicization look like pure presidential self-interest.


Practice regarding FBI directors doesn’t go back all that far, because J. Edgar Hoover ran the department from 1924 to 1972, ultimately dying in office. Hoover was too powerful and knew too much to be fired.


In reaction, Congress adopted a law in 1976 that limited the director to a 10-year term. The law doesn’t place any limits on presidential power to fire the director. Arguably, law enforcement is so central to the core constitutional power of the executive that it would violate the separation of powers if Congress tried to take away the president’s authority to remove the chief federal law enforcement officer.


At the same time, however, it’s anomalous in a rule-of-law system for law enforcement to be too responsive to the political whims of the elected executive. It’s just very risky to allow a country’s most powerful elected official to control the appointment of key law enforcement officers —- in part because of conflicts of interest like the one raised by the Comey firing. As a result, the vast majority of well-functioning democracies professionalize the investigative role, rather than politicizing it.


That’s been the unwritten norm in the U.S. — one might almost say, a part of our unwritten, small-c constitution, though not of the written, big-C Constitution. Of the four Senate-confirmed directors before Comey, all served under presidents of both parties. Three served until their terms ended or they voluntarily retired. One, Robert Mueller, got a special two-year extension.


The exception was William Sessions. Sessions, initially appointed by President Ronald Reagan, was fired by Clinton after an investigation by the Office of Professional Responsibility of the Department of Justice found that he’d used FBI planes to visit friends and relatives. Clinton tried to get Sessions to resign in order not to have to break precedent and fire him. But Sessions refused, and Clinton pulled the trigger and fired him anyway.


Trump alluded indirectly to the Sessions firing in his message to Comey when he said “you are not able to effectively lead the bureau.” This echoed Clinton’s language when he said that Sessions could “no longer effectively lead the bureau.”


By implication, Trump was saying that he has as much right to fire Comey as Clinton did to fire Sessions. In practice, there’s a big difference between Sessions’s ethics violations, which were documented by George H.W. Bush’s Department of Justice, and Comey’s admittedly highly problematic management of the investigation of Hillary Clinton.


Comey may arguably have acted unethically by announcing the reopening of the Clinton email investigation shortly before November’s election — but Trump didn’t say so, and surely he’s the last person in the world who would make that claim. The firing of Comey is blatantly political. The bottom line is presumably that the Trump administration knows it can’t control Comey, and so it doesn’t trust him.


It seems to me, for what it’s worth, that Comey should have resigned after Trump’s election to avoid the appearance that he had politicized his position to the benefit of the candidate who won. I’m not writing to mourn his tenure.


Yet Comey’s act of politicization doesn’t justify Trump’s decision to make the firing of the FBI director into a political act. It’s a classic case of two wrongs not making a right.


And it’s profoundly troubling that a president whose administration is already under investigation on multiple fronts would take such an action. Whoever is appointed to run the FBI permanently will be seen as beholden to the president who appointed him or her. That will make any decision not to pursue investigations into the president look politically motivated and illegitimate.


The erosion of the independence of law enforcement is thus a blow to the unwritten constitutional norm of political neutrality. It doesn’t violate the separation of powers. But it violates a norm that in its own way is almost as important.


Noah Feldman is a Bloomberg View columnist. He is a professor of constitutional and international law at Harvard University and was a clerk to U.S. Supreme Court Justice David Souter. Readers may email him at nfeldman7@bloomberg.net.