It has long been presumed that special counsel Robert Mueller’s investigation was based on an original sin: President Donald Trump’s May 2017 decision to fire James Comey as director of the FBI.

But Mueller’s long-awaited report argues that an action of greater potential criminality on the president’s part was one that came a month later, on June 17.

On that Saturday, Trump twice telephoned his then-White House counsel Donald McGahnfrom Camp David and told McGahn to get rid of Mueller himself.

Though one of the qualifications required of those who work for this president is a high tolerance for his impulsive and questionable behavior, the White House counsel was shocked when Trump told him, “Mueller has to go.”

McGahn refused to carry out an order that, according to the special counsel’s report, he believed was on par with President Richard Nixon’s infamous “Saturday Night Massacre.”

“McGahn considered the President’s request to be an inflection point and he wanted to hit the brakes,” the special counsel’s report said.

From there, McGahn “called his lawyer, drove to the White House, packed up his office, prepared to submit a resignation letter with his chief of staff, told (then-White HouseChief of Staff Reince) Priebus that the President had asked him to ‘do crazy s---.’ ”

Ultimately, McGahn decided to stay in his job for another 16 months, and Trump dropped his efforts to remove Mueller. But the relationship between the president and his White Housecounsel remained strained and was made more so when it became known that McGahn had given 30 hours of testimony to Mueller.

The broad outlines of Trump’s effort to get rid of Mueller had been known, though the president himself had lied when The New York Times first reported it in January 2018. “Fake news, folks,” Trump said at the time.

“Those denials are contrary to the evidence and suggest the President’s awareness that the direction to McGahn could be seen as improper,” the 448-page Mueller document noted.

The Mueller report provides not only a striking and much fuller narrative of Trump’s actions but also shows why they might put his presidency in jeopardy. Though Trump would not be criminally prosecuted while in office, House Judiciary Committee Chairman Jerrold Nadler, D-N.Y., saidat a news conference Thursday that theMueller report showed “clear evidence” that Trump engaged in obstruction of justice and that impeachment is “one possibility.”

Mueller also rejected claims by the president andhis allies that he is shielded from obstruction-of-justice laws, writing: “The Constitution does not categorically and permanently immunize the president.”

One reason Trump’s effort to fire Mueller could be so damaging is what it says about the president’s frame of mind; as the report notes, obstruction is a crime only if the perpetrator acted “knowingly and dishonestly” or “with an improper motive.”

When Trump fired Comey, according to the report, he believed — based on assurances from the FBI director himself — that he was not personally under investigation. But as a result of Comey’s dismissal, the probe did indeed pivot toward Trump himself, which the president learned in a Washington Post article published three days before he demanded that McGahn fire Mueller.

So began what the report described as the second of “two distinct phases reflecting a possible shift in the President’s motives.”

“The President became aware that investigators were conducting an obstruction-of-justice inquiry into his own conduct. That awareness marked a significant change in the President’s conduct and the start of a second phase of action,” the report said. “The President launched public attacks on the investigation and individuals involved in it who could possess evidence adverse to the President, while in private, the President engaged in a series of targeted efforts to control the investigation.”

In stopping Trump from firing Mueller, McGahn no doubt thought he was saving the president from himself, and — not incidentally — the White House counsel’s own reputation.

Standing firm to rescue Mueller’s job was the right thing to do, and an act of defiance that we see too rarely among those with whom Trump surrounds himself. But the fact that McGahn had to do it could be the most damning evidence of all to be produced by the investigation — and the dogged investigator — that the president could not make go away.

Karen Tumulty covers national politics.