Whatever else one might say about the just-concluded 2016 presidential election, one thing is certain: FBI Director James Comey played an outsized and exceptionally inappropriate part. His highly prejudicial announcement on October 28, just eleven days before the election, that he had reopened an investigation into Hillary Clinton’s private e-mail server ensured that the final critical days of the campaign were taken up with innuendos and suppositions set off by his action.

When Comey then announced on November 5, just two days before the election, that upon further review he had again found no basis to believe that Clinton had committed any crime, it only underscored the impropriety of his October 28 announcement. Had he conducted the review in confidence, as Justice Department rules require, the entire matter would have been resolved without interfering with the election. As it was, his October 28 announcement dramatically shifted the trajectory of the campaign, deflected attention from Donald Trump’s own considerable troubles, and inevitably influenced the choices of many early voters.

Comey made the renewed investigation public against Justice Department policy and rules, and over the objections of the attorney general and several other Justice Department officials, even though he had not even seen the new evidence, much less determined that it hinted at any wrongdoing on Clinton’s part. The announcement predictably played right into the hands of Trump, who immediately took the occasion to repeat his charge that Clinton should be locked up. None of this should have happened; under long-standing Justice Department practice, Comey should have kept silent about the fact of further investigation, especially so close to an election.

Whether Comey’s imprudent intervention changed the outcome of the presidential election, the damage to the integrity of both the political and criminal processes has been done. The criminal process has been politicized, and the political process has been tainted by misuse of official power. The question that remains is what should happen now. At a minimum, the Justice Department policies that Comey violated must be strengthened and formalized to ensure that this never happens again.

The first rule Comey disregarded requires law enforcement officials to avoid public comment on ongoing investigations. The Justice Department’s guidelines on “Disclosure of Information Concerning Ongoing Investigations,” set forth in the US Attorneys’ Manual, provide that, as a general matter, officials “shall not respond to questions about the existence of an ongoing investigation or comment on its nature or progress.” The rule recognizes that on “matters that have already received substantial publicity” or where public safety requires, “comments about or confirmation of an ongoing investigation may need to be made.” But that exception is plainly a narrow one. The general no-comment rule is designed to prevent…