TWENTY MONTHS after the Justice Department’s inspector general sent a referral about Andrew McCabe to the U.S. Attorney’s Office for the District of Columbia, the fired FBI deputy director remains in limbo about whether criminal charges will be brought against him. It is a pretty straightforward case, and there has been no good explanation as to why U.S. Attorney Jessie K. Liu has allowed it to linger. It now appears Ms. Liu may be leaving her office for another job in the Trump administration; before she goes, she needs to resolve this matter — guided, we hope, by the law and not the political agenda of a vengeful president.

Mr. McCabe, a 21-year FBI veteran fired a day short of his retirement in 2018, has been a frequent target of President Trump, who has called him “a major sleazebag” and celebrated his firing as “a great day for the hard working men and women of the FBI.” His dismissal after a distinguished career coincided with the inspector general’s findings that Mr. McCabe violated Justice Department policy by authorizing an aide to talk with a Wall Street Journal reporter about the agency’s probe into the Clinton Foundation and that he displayed a “lack of candor” in discussing the matter afterward inside the Justice Department.

Whether Mr. McCabe — who has denied any wrongdoing — intentionally lied and should be charged with perjury was to be determined by Ms. Liu’s office. The subsequent handling of the case has been, at best, questionable. Investigation dragged on for so long that one grand jury’s term expired. Another grand jury met in September to consider charges without issuing an indictment, fueling speculation that grand jurors wouldn’t sign on to a prosecution.

In November, U.S. District Judge Reggie B. Walton, presiding over a lawsuit brought by the nonprofit group Citizens for Responsibility and Ethics in Washington seeking records about the internal inquiry into Mr. McCabe, criticized prosecutors for their handling of the case, saying the uncertainty was unfair to Mr. McCabe and to the public. “This is not a hard case. I was a good prosecutor for a long time. Deciding whether or not you’re going to charge someone with false statements or perjury is not that hard, factually or legally — maybe politically, but not factually or legally,” said the exasperated judge. It is hard not to read between the lines and see prosecutors who realize they have a bad legal case but won’t close it out for fear of angering the president.

Mr. Trump sees nothing wrong with using the justice system to try to advance his political interests. In the McCabe case, Ms. Liu must demonstrate that she understands her obligations are strictly to the rule of law.

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