Last June, Donald Trump ordered White House Counsel Don McGahn to fire Special Counsel Robert Mueller, according to the New York Times. McGahn threatened to resign rather than fire Mueller, so Trump backed down.

Trump had given McGahn an illegal order. The president was asking the White House Counsel to conspire with him to obstruct justice. McGahn knew he had a legal duty to refuse an illegal order, even if it was the president doing the ordering. McGahn was “concerned that firing the special counsel would incite more questions about whether the White House was trying to obstruct the Russia investigation,” according to the Times.

The federal obstruction of justice statute punishes anyone who “corruptly … influences, obstructs, or impedes or endeavors to influence, obstruct, or impede the due and proper administration of the law under which any pending proceeding is being had before any department or agency of the United States.”

Mueller had been appointed in May, a month before Trump ordered McGahn to fire him. Mueller’s charge was to investigate “any links and/or coordination between the Russian government and individuals associated with the campaign of President Donald Trump” as well as “any matters that arose or may arise directly from the investigation.”

Trump cited three reasons for wanting Mueller fired: that Mueller resigned from Trump’s golf club because of a dispute about dues; that Mueller’s old law firm had represented Trump’s son-in-law Jared Kushner; and that Trump had interviewed Mueller to replace James Comey as FBI director.

But none of those purported justifications pass the straight face test, and apparently McGahn didn’t think so, either. Trump “spoke with a number of friends and advisers who convinced him that Mueller would dig through his private finances and look beyond questions of collusion with Russians,” the Washington Post reported. “They warned that the probe could last years and would ruin his first term in office.”

Moreover, it was public knowledge in May 2017 that Mueller was investigating possible obstruction of justice by Trump for firing FBI director James Comey.

Trump sought to shut down the investigation by firing Mueller. That means he endeavored to obstruct the investigation with corrupt intent, as specified by the obstruction of justice statute.

The president’s illegal order to McGahn corroborates the growing evidence of obstruction of justice by Trump.

Mueller had been appointed after Trump unlawfully fired Comey, who was conducting an investigation into possible collusion between the Trump campaign and Russia. Trump had demanded loyalty from Comey and asked the FBI director to end the investigation of former National Security Adviser Michael Flynn in connection with the Russia probe. When Comey failed to do so, Trump fired him.

The next day, Trump bragged to Russian officials in the Oval Office, “I just fired the head of the FBI. He was crazy, a real nut job,” adding, “I faced great pressure because of Russia. That’s taken off.” The day after he talked to the Russian officials, Trump told NBC’s Lester Holt, “When I decided to just do it [fire Comey], I said to myself … this Russia thing with Trump and Russia is a made-up story.”

Trump was admitting he had a corrupt motive for firing Comey: to stop the Russia investigation. That constitutes obstruction of justice.

Indeed, Philip Allen Lacovara, former Justice Department deputy solicitor general and counsel to Watergate special prosecutors Archibald Cox and Leon Jaworski, wrote in the Washington Post:

Comey’s statement lays out a case against the president that consists of a tidy pattern, beginning with the demand for loyalty, the threat to terminate Comey’s job, the repeated requests to turn off the investigation into Flynn and the final infliction of career punishment for failing to succumb to the president’s requests, all followed by the president’s own concession about his motive. Any experienced prosecutor would see these facts as establishing a prima facie case of obstruction of justice.

Trump also took aim at Deputy Attorney General Rod Rosenstein, who had appointed Mueller special counsel after Attorney General Jeff Sessions recused himself from the Russia investigation. “Another option that Mr. Trump considered in discussions with his advisers was dismissing” Rosenstein “and elevating the Justice Department’s No. 3 official, Rachel Brand, to oversee Mr. Mueller,” the Times reported.

If Trump were to fire Rosenstein, we would have a “Saturday Night Massacre” redux. When, in 1973, special prosecutor Archibald Cox was investigating Watergate, then-President Richard Nixon ordered the attorney general and deputy attorney general to fire Cox. They resigned instead, much like McGahn threatened to do when Trump ordered him to fire Mueller. Cox was then fired by Solicitor General Robert Bork. A judge later determined that Cox’s firing was unlawful.

Trump was also furious with Sessions for recusing himself. The president expected Sessions to have his back. Although the attorney general is not the president’s personal lawyer, Trump sought complete loyalty from Sessions, at one point, demanding to know, “Where’s my Roy Cohn?”

Roy Cohn was a lawyer for Sen. Joe McCarthy, whose anti-communist witch hunts in the 1950s ruined countless lives. McCarthy was eventually censured and Cohn was disbarred. Cohn, who had been Trump’s personal lawyer and friend for many years, was notorious for doing whatever it took to please his boss.

Trump, who demands loyalty from everyone around him, is probably increasingly nervous these days, as Mueller has secured the cooperation of Flynn and former Trump foreign policy adviser George Papadopoulos, both of whom face sentencing after pleading guilty to lying to the FBI.

Mueller has interviewed McGahn twice. He has spoken to eight members of McGahn’s office, as well as Comey, Sessions and CIA Director Mike Pompeo. He will also reportedly interview former Chief White House Strategist Steve Bannon, who, perhaps more than anyone, has extensive incriminating information about the president.

Trump, who denied he tried to fire Mueller — deploying his customary retort, “fake news” — has offered to sit down with Mueller and answer questions. One can be guilty of lying to the FBI without being under oath. If Trump’s lawyers convince the president to reconsider, Mueller could compel his sworn testimony before the grand jury. Both avenues are fraught with peril for the serially lying president.