Andrew Coan is a professor of law at the University of Arizona and author of the forthcoming book, “Prosecuting the President: How Special Prosecutors Hold Presidents Accountable and Protect the Rule of Law.”

On June 8, attorney general-nominee William P. Barr sent the Justice Department an unsolicited memo attacking special counsel Robert S. Mueller III’s obstruction-of-justice investigation as “fatally misconceived.” It is Barr’s legal theory that is misconceived. From all appearances, Mueller’s investigation rests on exactly the same foundation as the case against President Richard M. Nixon. If Nixon was a crook, there is a real possibility that President Trump is, too.

The crux of Barr’s argument, spelled out in a 19-page memo first reported by the Wall Street Journal, is that a president cannot obstruct justice when carrying out his normal constitutional responsibilities, such as hiring and firing officials and directing federal law enforcement operations. But that is just what Nixon was doing when he attempted to shut down the FBI investigation of Watergate in 1972. When this came to light two years later, no one seriously argued that Nixon had acted lawfully. Rather, he earned himself a one-way ticket out of town.

The story goes like this. On June 17, 1972, six operatives hired by Nixon’s reelection campaign broke into Democratic National Committee headquarters at the Watergate office complex. Six days later, Nixon’s chief of staff, H.R. “Bob” Haldeman, broached the subject with his boss. “Now, on the investigation,” Haldeman said. “You know, the Democratic break-in thing. We’re back . . . in the problem area because the FBI is not under control. . . . They’ve been able to trace the money . . . through the bank . . . and it goes in some directions we don’t want it to go.”

The money in question was that paid to the Watergate burglars, which the FBI was on the verge of tracing back to Nixon’s campaign committee. It did not take a genius to see that this had the makings of a political calamity. As usual, however, Haldeman had a plan: The president would order the CIA to call FBI Director L. Patrick Gray “and just say, ‘Stay the hell out of this. This is business here we don’t want you to go any further on.’ ” As Haldeman saw it, this was the administration’s only play.

“All right. Fine,” Nixon said.

Nixon wrapped up the conversation by giving Haldeman a script for his call with the CIA: “When you get these people in, say: ‘Look, the problem is that this will open the whole Bay of Pigs thing.” Nixon trailed off. “Don’t lie to them. . . . Just say this is sort of a comedy of errors, bizarre, without getting into it. . . . They should call the FBI in and say that we wish for the country, ‘Don’t go any further into this case, period!’ ”

This was a complete fabrication. Nixon was not actually worried about national security or foreign relations. He was simply trying to protect his closest aides from criminal investigation and himself from untold political damage.

This episode poses an almost perfect test of Barr’s argument that a president cannot obstruct justice while exercising his constitutional responsibilities. As head of the executive branch, the president unquestionably possesses constitutional authority to supervise the FBI and the CIA. If the Constitution permits a president to exercise this authority for any reason, as Barr contends, there could be nothing improper about Nixon’s actions. Yet when Nixon released a transcript of this conversation on Aug. 5, 1974, it immediately became known as “the smoking gun.” He resigned the presidency three days later.

Now, almost 45 years on, a very similar case can be sketched. Trump asked FBI Director James B. Comey to end the investigation of national security adviser Michael Flynn. Trump then fired Comey and applied unrelenting pressure on Attorney General Jeff Sessions to end the Mueller investigation. When Sessions wouldn’t budge, Trump replaced him with an acting attorney general whose main qualification for office was his personal loyalty to the president. Finally, Trump nominated Barr, a public — and it now turns out, an even fiercer private — critic of the Mueller investigation.

All of these actions fall within a president’s constitutional responsibilities. But if Trump took them for the purpose of protecting himself and his friends from legal jeopardy, his conduct is nearly identical to Nixon’s, and he should meet the same fate. If the rule were otherwise, the president could never be investigated or removed from office for actions within the scope of his official authority — not even for accepting a bribe or acting in the service of a foreign power. That cannot be true.

Of course, we do not yet know all the facts in this case. But that is why Mueller’s investigation is so important. Barr’s call to shut down the obstruction side of that investigation — and to block the special counsel from questioning Trump on the subject — is legally baseless and practically dangerous. It would go far toward placing the president above the law. Senators should think long and hard before confirming an attorney general who holds such views.

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