Matthew G. Whitaker attends a roundtable discussion with foreign liaison officers in August at the Justice Department in Washington. (Allison Shelley/Reuters)

Harry Litman teaches constitutional law at the University of California at San Diego and practices law at the firm Constantine Cannon. He was U.S. attorney for the Western District of Pennsylvania from 1998 to 2001 and deputy assistant attorney general from 1993 to 1998.

How serious is the forced resignation of Jeff Sessions and the installation of his chief of staff, Matthew G. Whitaker, as acting attorney general? And what does it portend for the Mueller probe and related investigations?

The first question is easy: It is as serious as a heart attack. Whitaker’s appointment, which President Trump effectuated before all of the midterm election results were even final, immediately divested Deputy Attorney General Rod J. Rosenstein of his oversight authority of special counsel Robert S. Mueller III’s investigation.

Rosenstein had bent occasionally in response to the gale-strength pressures on him from the White House, but had never broken, performing a tremendous service to the department and the country in safeguarding the integrity of Mueller’s work.

Whitaker, it is clear, is a far less trustworthy guardian. Indeed, he already expressed opposition to the Mueller probe in an 2017 op-ed for CNN, in which he wrote: “Mueller has come up to a red line in the Russia 2016 election-meddling investigation that he is dangerously close to crossing.”

And The Post previously reported that in an 2017 CNN interview, Whitaker speculated about a scenario in which a new attorney general doesn’t fire Mueller but instead “reduces his budget to so low that his investigation grinds to almost a halt.” That scenario is now a real — and chilling — prospect.

Moreover, Whitaker is a fundamentally political actor. He co-chaired Rick Perry’s 2012 presidential campaign and ran unsuccessfully as a Republican for the Senate in 2014 as well as state treasurer in Iowa in 2002. That’s a significant and foreboding fact in this setting. It’s not hard to envision that in some general way, he will have Trump’s back, even though it would be a stark dereliction of duty.

Other potential Sessions replacements, such as former associate attorney general Rachel Brand or Solicitor General Noel Francisco, belong to a professional class of D.C. lawyers with high institutional regard for the Justice Department and a reluctance to throw bombs. But Whitaker’s radical credentials could be a political boon for the president.

And on the topic of radical bomb-throwers, Whitaker has characterized Marbury v. Madison, the Promethean foundation of judicial review in the United States and probably the most important judicial decision ever rendered, as one of the worst decisions in the Supreme Court’s history. His critique was that “the courts are supposed to be the inferior branch of our three branches of government” whereas Marbury established the court as the final arbiter of constitutional issues. He similarly trashed “all New Deal cases that were expansive of the federal government.” These views are a stunning and deeply inappropriate set of opinions for the nation’s chief law enforcement officer.

So what havoc could Whitaker wreak on the Mueller probe and related cases that Mueller already has parceled out to other prosecutors, such as the Michael Cohen probe?

It’s a safe bet that he will not seek to fire Mueller or bring the investigation to an immediate close. A former U.S. attorney, Whitaker understands that Mueller is the gold standard and that smothering the investigation would be a political nonstarter. Moreover, unless he tries to replace the governing regulations, he can remove Mueller only for good cause, an impossible standard to satisfy in Mueller’s case.

But that leaves a long list of possible mischief that could cripple the probe. First, Whitaker has now assumed immediate control over Mueller’s budget and personnel matters. Second, he has authority to disapprove of requests to change or expand the probe’s jurisdiction.

Third, the regulations state that Mueller is not subject to the day-to-day supervision of the attorney general or anyone else in the Justice Department, but they permit the attorney general to request that Mueller provide an explanation for any investigate step. The attorney general can then halt such a step if he concludes that it is inappropriate or unwarranted under departmental practices. Properly read, that’s a narrow standard; but as a practical matter it will be Whitaker’s call how to read it.

Finally, Whitaker now assumes great power over any report Mueller produces, and it has been widely assumed that Mueller’s obstruction report would be ready not long after the midterms. He can decide whether the report’s release — which Team Trump has already indicated it would fight — would be “in the public interest.” Whitaker probably couldn’t bottle up or bury the report indefinitely, but he might be able to suppress it long enough to blunt serious political fallout for the president.

And of course, as acting attorney general, Whitaker now becomes the ultimate boss over the prosecutors in the Southern District of New York and the Justice Department’s National Security Division, where cases loosely related to the Mueller probe have been brought. Those assignments were a prudent divestiture to insulate the cases from any direct attack on Mueller himself, but Whitaker has even more direct authority over them than over Mueller himself.

In short, Trump has made his post-midterm move to bring the Mueller investigation under control, and it’s a broad one. The threat to Mueller’s ability to complete a comprehensive investigation is material and might yet be mortal.