Nor is an investigation of this magnitude limited to a single office. Each case generates leads — threads of inquiry, like an interview or surveillance of an intelligence officer who might be traveling to another state — that span the country. When this happens, F.B.I. agents don’t hop on a plane. Rather, the “home” office for the case (called the “office of origin”) will send a lead to the field office with jurisdiction over that area.

Mr. Mueller’s investigation is more closely held than most, but its tentacles have already clearly spread to other field offices — consider the investigation against President Trump’s personal lawyer, Michael Cohen, run out of the Southern District of New York office, or the plea deal of a California man, Richard Pinedo, who assisted Russia in executing its disinformation campaign on social media. Field offices are evaluated in part based on their success in following through on leads and making cases that result in arrests and convictions. No case agent worth their salt would remain quiet if their cases were closed in the face of a continuing threat. To “shut down” the investigation at this point would require not just a face-off with Mr. Mueller but also with special agents in charge of multiple field offices with a vested interest in seeing their responsibilities through, and possibly even a battle with the F.B.I. director, Christopher Wray.

Finally, many of the most critical parts of the Russia investigation have already entered the judicial system. Mr. Mueller has already indicted numerous individuals and companies and secured several convictions. Five members of the Trump campaign have pleaded guilty to federal charges and are actively cooperating with prosecutors. Grand juries have seen evidence, judges have signed off on warrants, and there are likely subpoenas and indictments waiting in the wings. Whatever happens now, there is no doubt that the F.B.I. has already collected hundreds of thousands of pages of evidence in the form of documents, interviews, electronic surveillance and foreign intelligence shared by our allies that are stored in the F.B.I.’s tamper-proof system and cannot be destroyed.

This isn’t to say that a Rosenstein replacement couldn’t do any damage — he could try to starve the investigation of resources, for example, or withhold approval for investigative steps that have yet to be taken. But considering that the potential replacements for Mr. Rosenstein — Solicitor General Noel Francisco or the head of the Office of Legal Counsel, Steven Engel — have very little experience in counterintelligence and criminal matters, they would face an uphill battle justifying those decisions against seasoned prosecutors and in the face of evidence warranting otherwise.

Under the special counsel regulations, moreover, any such decision would be required to be reported to Congress. With the precedent set by Devin Nunes on the House Intelligence Committee, if Congress changes hands, it’s going to be very difficult for the president to try to block obvious attempts to obstruct justice from seeing the light of day.