THE NATION ought to be relieved that President-elect Donald Trump has decided not to press his campaign pledge to criminally investigate rival Hillary Clinton for her handling of email while secretary of state and for the activities of the Clinton Foundation. A drawn-out probe, fueled by Mr. Trump from the White House, would invariably become a political circus, take on the overtones of vendetta and deepen the wounds of the election. It would represent a continuation of the reckless “lock her up” chants by Mr. Trump’s campaign crowds, a mantra that suggested a Trump administration would run roughshod over the rule of law.

It would also fly in the face of the conclusion already drawn by FBI Director James B. Comey after a thorough investigation of Ms. Clinton’s use of a private email server while secretary of state. Mr. Comey said that no reasonable prosecutor would bring a case against Ms. Clinton based on the available evidence.

With the presidential campaign ended, Mr. Trump seems to have concluded that the legal pursuit of Ms. Clinton or former president Bill Clinton would be a legal and political loser. “It would be very, very divisive for the country,” he told the New York Times on Tuesday. “My inclination,” he said, “would be, for whatever power I have on the matter, is to say let’s go forward. This has been looked at for so long, ad nauseam.”

But even in his welcome rethinking, it was not clear that Mr. Trump understands the principle of justice insulated from political control. His statement contained only a glimmer of recognition that, as president, it should not be for him to decide whether criminal prosecutions are undertaken. The law enforcement system and the U.S. attorneys who investigate and prosecute federal crimes are supposed to be independent, free from interference by the White House or anyone else. The president’s role is to appoint an attorney general and set broad political priorities.

In the case of Ms. Clinton’s emails, Mr. Comey has already made a determination; Mr. Trump’s statement suggested he will not, improperly, seek to reverse it once in office. Separately, there may be ongoing investigations into the Clinton Foundation. If so, they should be left to the professionals at the FBI and career prosecutors at the Justice Department.

President-elect Donald Trump's supporters often chanted "lock her up" when he discussed his opponent, Hillary Clinton, at campaign rallies. But despite repeatedly promising to appoint a special prosecutor to investigate her, it looks like Trump might not pursue Clinton after all. (Peter Stevenson/The Washington Post)

In his career as a brassy and pugilistic tycoon, Mr. Trump often gave the impression he saw the law as just another tool for advancing his personal interests or retaliating against his foes. Once he sits in the Oval Office, he must behave — and think — differently. His apparent decision not to pursue Ms. Clinton is a step in the right direction. But given his past behavior and campaign statements, it remains incumbent on Mr. Trump to show that he fully accepts the U.S. constitutional mandate of impartial justice and a political system ruled by law.