Unless they are on a secret Washington dinner party circuit, there are no hints that Robert Mueller and Nancy Pelosi are friends.

Sure, they overlapped 20 years ago with Mueller serving as US attorney for the northern district of California and Pelosi in her second decade representing San Francisco in Congress. And, like all prominent Washington public officials, they shared TV green rooms when Mueller was FBI director and Pelosi was on her first circuit as House speaker.

Despite their differing persona, these two past-normal-retirement-age figures born in the early 1940s boast important affinities. In a political age defined by corrosive cynicism, they both share an old-fashioned belief in institutions whether they are Mueller’s justice department or Pelosi’s House of Representatives.

That institutional faith pinpoints Mueller’s target audience for his nine-minute coda to this tenure as special counsel. In an extreme example of narrowcasting, masked by the careful legalistic language, Mueller was speaking directly to Pelosi.

His implicit message: my institution (the justice department) cannot indict a sitting president. But your institution (the House) can vote to impeach Donald Trump and mandate a Senate trial for obstruction of justice and possible other “high crimes and misdemeanors”.

Much of Mueller’s statement served as an explanation of the constraints that he felt because of a justice department legal interpretation that he cannot indict a sitting president. As Mueller bluntly put it: “A special counsel’s office is part of the Department of Justice, and by regulation, it was bound by that department policy.”

Then Mueller, in the same procedural step-by-step tone of a legal indictment, went on to deliver one of the most important sentences of his tenure as special counsel: “The constitution requires a process other than the criminal justice system to formally accuse a sitting president of wrongdoing.”

Mueller could not have been clearer about impeachment if he had stepped before the cameras with a scarlet I pinned to his suit.

Up to now, the debates over impeachment roiling the House Democratic caucus have either been political (“It will cost us seats in 2020”) or moral (“How can we tell our grandchildren that we did nothing in the face of Trump’s lawless behavior?”).

But Mueller, in his subtle, understated fashion, tried to break this Democratic stalemate. He offered a new argument and perhaps the only one that could possibly trump Pelosi’s hard-won political caution. What Mueller was saying, in effect, was that the constitution and the institutional legitimacy of Congress as an independent body require commencing impeachment hearings.

Of course, Mueller’s understanding of institutional imperatives means that he would be about as reluctant an impeachment witness as a straight-arrow cop testifying in a police corruption scandal.

Mueller’s other message to Pelosi on Wednesday was to try to convince her not to compel his congressional testimony. He broadly hinted that he would answer all questions with boring-for-television lines such as: “You will find my answer on page 84 of the second section of my report.”

There is a common misunderstanding about an impeachment inquiry. The dominant impression seems to be that the House judiciary committee would open hearings on a Tuesday, vote out an impeachment resolution on a Wednesday and have it pass the House on a party-line vote (plus anti-Trump GOP apostate Justin Amash) on Thursday. Then, a week later, Mitch McConnell and company would find Trump “not guilty” in the Senate.

Impeachment hearings can and should be as deliberate as the Mueller inquiry itself

But impeachment hearings can and should be as deliberate as the Mueller inquiry itself. Unlike 1974, when the Richard Nixon impeachment hearings built on the earlier dramatic public testimony of the Senate Watergate committee, this time around the House judiciary committee’s efforts would need to be both investigative and prosecutorial. Witnesses such as the former White House counsel Don McGahn and Hope Hicks, the former communications director, would have a hard time dodging congressional subpoenas for a constitutionally sanctioned impeachment hearing no matter how ardently Trump claimed executive privilege.

Mueller had a narrow mandate, but there are no such limitations on impeachment hearings.

Trump’s shameless profiteering in the White House, with some of the money coming from foreign sources, may well violate the “emoluments clause” of the constitution. His conduct in office also raises grave national security questions from secret meetings (with no American note-takers) with Vladimir Putin to Jared Kushner’s dubious top-level access to secret documents.

Mueller may be faulted for being too punctilious in his fidelity to justice department rules and precedents. But he followed what he saw as the path dictated by integrity to the end. Now it is up to Pelosi also to transcend politics – and do what the constitution demands.