POLITICAL TENSION is running high in the United States, extraordinarily so, we’d say. And so it behooves everyone in a position of official responsibility to do everything he or she possibly can to help maintain stability — while avoiding all avoidable provocations — until the bitter competition between Hillary Clinton and Donald Trump runs its ugly course on Nov. 8.

That is the context for Friday’s announcement by James B. Comey, director of the Federal Bureau of Investigation, that his agency is again looking intoMs. Clinton’s private email server in light of newly discovered emails “that appear to be pertinent to the investigation.” Mr. Comey may have had good reason to inform Republican committee chairmen in Congress of the review, but his timing was nevertheless unfortunate, given its potential to affect a democratic process in which millions of people are already voting.

What might his reason be? On the merits, Ms. Clinton erred by using a private email server for her official communications as secretary of state — though as we have previously argued, the matter has been greatly overblown. According to the previous FBI review, the small amount of classified material that moved through Ms. Clinton’s private server was not clearly marked as such, and no harm to national security has been demonstrated.

The FBI conducted a thorough investigation for any prosecutable offenses, especially any involving the transmission of classified information. Mr. Comey rightly recommended against bringing charges; he told his staffthat the decision was “not a cliff-hanger.” In deference to the reality that the target of the inquiry was a major-party nominee for president, he gave the public a summary of the facts and law behind his decision.