Specifically, the regulations are written for appointing someone to run a criminal investigation, but Mr. Mueller was inheriting a counterintelligence inquiry. The regulations also envision appointing someone from outside the Justice Department as special counsel, but Mr. Durham is the sitting U.S. attorney for Connecticut.
Because Mr. Durham was not appointed pursuant to the special counsel regulation, it is possible the next attorney general could rescind Mr. Barr’s directive that special counsel rules would apply to him, then end his inquiry without any finding of misconduct. That was also a theoretical possibility for Mr. Mueller, but it did not matter for most of the Russia investigation because Mr. Rosenstein himself had voluntarily adopted the rules and remained in charge.
Still, said Samuel Buell, a Duke University law professor and former federal prosecutor, “I suppose the calculation is that there is a political cost” if a Biden administration attorney general were to try to shut down Mr. Durham’s work as a special counsel.
Mr. Barr had assigned Mr. Durham last year to conduct a “review” of actions taken by the F.B.I. and other national security officials in the early stages of the Russia investigation. The Justice Department later said his work had evolved into a criminal investigation, and Mr. Barr’s letter to Congress said that status was “ongoing.”
But while Mr. Durham has looked into a number of issues in search of evidence to bolster Mr. Trump’s oft-stated declaration that a “deep state” plotted to sabotage him, it is not clear what, if anything, he has found. To date, the only criminal prosecution he has brought was by striking a plea deal with Kevin Clinesmith, a former lower-level F.B.I. lawyer. He had doctored an email from the C.I.A. when the bureau was preparing to apply for renewal of a wiretap order targeting a former Trump campaign aide with links to Russia, Carter Page.
The alteration of the C.I.A. email by Mr. Clinesmith, who has not yet been sentenced, prevented an F.B.I. colleague from realizing that the application — and prior iterations — omitted a relevant fact: Mr. Page had discussed with the C.I.A. some of his interactions with Russians, potentially making his pattern of such contacts look less suspicious. But a separate investigation by the Justice Department’s inspector general, Michael E. Horowitz, uncovered that issue, along with other ways the F.B.I. botched the applications — not Mr. Durham.
Expectations had built that Mr. Durham would announce something important before the election, in part because Mr. Barr had stoked them by saying he did not think a department policy against taking actions that could affect an upcoming election applied to Mr. Durham’s inquiry.