Attorney General Jeff Sessions continues to resist calls to recuse himself from Department of Justice investigations into Russian connections with the Trump campaign and later with the transition team. He should stop resisting and step aside. He has a blatant conflict of interest.

The problem is not just that Sessions was appointed to his current position by Trump and is a close political ally of the President—although that alone probably justifies recusal. The problem is not just that Sessions himself played a prominent role in the campaign. The problem is that he headed the Trump campaign’s national security advisory committee, and in that capacity spoke out publicly in favor of trying to forge closer ties with Russia. Sessions later served on the executive committee of Trump’s transition team.

The New York Times reported yesterday that Trump campaign officials had “repeated contacts with senior Russian intelligence officials.” The Times did not name its sources, described as “four current and former American officials,” and those sources did not name the campaign officials involved, aside from one of Trump’s former campaign chairs, Paul Manafort. So even assuming the story is correct, Sessions may not himself have been in contact with Russian intelligence officials.  Furthermore the campaign officials who were in contact with the Russians may not have committed any crimes.  But the Department of Justice will need to decide whether and how to continue to investigate those questions, regardless whether—as appears increasingly likely—a separate, bipartisan congressional investigation is also launched into Russia’s involvement in the 2016 election.

The conflict of interest this creates for Sessions is extraordinary. At a minimum, Sessions himself—not just the man who appointed him—was deeply involved in the very campaign that is under investigation, he personally helped shape the campaign’s positions on national security, and he publicly supported a friendlier approach toward Russia.  It is hard to imagine how his own name—not just the name of the President who appointed him—can fail to surface in this investigation, and hard to imagine how the investigation can fail to touch on, if not Sessions’s own actions, at a minimum the actions of close political associates of his in 2016.

Attorney General Loretta Lynch faced loud calls to recuse herself from the Department of Justice investigation of Secretary Hillary Clinton’s handling of email after Lynch met privately on an airport tarmac with former President Bill Clinton. Lynch declined to do so, although she promised to accept any recommendation in the case from the FBI and senior career lawyers in the Justice Department. Views differed regarding whether that sufficed. The point is that the conflict Sessions faces here is far worse and far more direct. It is comparable to the conflict Lynch would have faced if, prior to her appointment as Attorney General, she had served in the State Department as Secretary Clinton’s chief advisor for email management.

There are no statutes specifying when the Attorney General should recuse himself from an investigation, but longstanding practice—as well as common sense—bars a prosecutor from supervising any investigation in which his or her fairness or objectivity could reasonably be questioned. Department of Justice regulations, for example, require recusal by a United States Attorney whenever “a personal interest or professional relationship with any of the parties [creates] an appearance of a conflict of interest or a loss of impartiality.”

That standard is obviously met here. The conflict is so strong that a good case can be made that the Department of Justice should appoint a special counsel to supervise the criminal investigation of Russian involvement in the 2016 election. But Sessions himself should not be involved even in that decision. He should recuse himself immediately. This is not a close call.

David Alan Sklansky is Stanley Morrison Professor of Law at Stanford Law School and a former federal prosecutor.