Despite the extraordinary success of the special counsel, Robert Mueller — or more likely because of it — faces his most serious threat yet: the forced resignation of Attorney General Jeff Sessions.

Early last year, Mr. Sessions, as a former Trump campaign official, recused himself from the Russia investigation, handing over its oversight to Deputy Attorney General Rod Rosenstein. As acting attorney general in that capacity, Mr. Rosenstein has overseen and vigorously defended the investigation, led by Mr. Mueller.

After requesting and receiving Mr. Sessions’s resignation on Wednesday, President Trump wasted no time in naming Matthew Whitaker, Mr. Sessions’s chief of staff, as acting attorney general, and shifted the oversight role from Mr. Rosenstein back to the attorney general’s office and its new acting head.

As ethics experts, we believe Mr. Whitaker should recuse himself from the investigation. If we have ever seen an appearance of impropriety in our decades of experience, this is it: a criminal subject president appointing his own prosecutor — one who has evidently prejudged aspects of the investigation and mused about how it can be hampered.