Attorney General William Barr three times now has tread on the dangerous ground of asserting that the president can assess his own guilt or innocence and, by extension, of the culpability of underlings as well.

Barr’s claims are meretricious nonsense.

The first assertion came by implication, but not fully stated, in Barr’s April 18 press conference before he released the Mueller report on Russian malfeasance. The next two, from his May 1 testimony to the Senate Judiciary Committee, were quite explicit.

Barr’s prepared press conference remarks ascribed “non-corrupt motives” to President Trump’s consideration of impeding Mueller’s probe, on the theory that Trump “was frustrated and angered by a sincere belief that the investigation was undermining his presidency, propelled by his political opponents, and fueled by illegal leaks,” even though “there was in fact no collusion.”

Yet frustration and anger provide no legal excuse for impeding a lawful investigation, and Barr has acknowledged the investigation was lawful. So how does this provide Trump any excuse?

In his May 1 testimony, Barr was more specific on two separate occasions. “If the president is being falsely accused … and he knew [the accusations against him] were false, and he felt this investigation was unfair, propelled by his political opponents, and was hampering his ability to govern, that is not a corrupt motive for [exercising constitutional authority] for replacing an independent counsel.”

In later testimony, Barr said: “With the president, who has a constitutional authority to supervise proceedings, if in fact a proceeding … was based on false allegations, the president does not have to sit there constitutionally and allow it to run its course. The president could terminate that proceeding and it would not be corrupt intent because he was being falsely accused, and he would be worried about the impact on his administration.”

Richard Nixon surely thought the Watergate investigation was a witch hunt. Yet when he wanted to fire the special prosecutor conducting it, Attorney General Elliot Richardson and his deputy both resigned. Then Congress immediately began impeachment proceedings.

Barr is a different sort of attorney general. His assertions are wrongheaded and possibly dangerous on multiple levels. The first is on principle alone: No man, not even the president, should be allowed to adjudge his own case. Otherwise, he’s a tyrant. The very notion of such power runs counter to the entire basis of America’s constitutionally limited government and its history of fighting a revolution to jettison a king.

Second, to the extent Barr is making more limited claims that a president has constitutional power to fire a constitutionally inferior officer (rather than to stop an entire investigation), he ignores the reality that even the powers granted by the Constitution are limited in both law and custom. Furthermore, any “intent” to impede an otherwise lawfully operating investigation on the basis of one’s own supposed innocence is to violate the first principle above. To violate a constitutional-historical principle as important as that one is inherently corrupt.

Third, Barr’s statements produce an absurd logical consequence. In practice, even if Trump knew with God-like omniscience that he was personally guilty of no legal offense, and is thus justified in Barr's mind to stop the investigation, he might also be killing an investigation into others — say, Paul Manafort, Michael Cohen, George Papadopoulos, Roger Stone, and more than a dozen Russian spies or other alleged criminals who worked illegally to undermine a U.S. election. Is Barr maintaining that this is also acceptable?

Remember, the Mueller investigation was only ever secondarily about Trump (or even Trump’s aides and relatives). The first official instruction given to Mueller was to “ensure a full and thorough investigation of the Russian government’s efforts to interfere in the 2016 election.” If Trump killed that proceeding even to keep it from “hampering his ability to govern,” he would be undermining the entire nation’s ability to protect itself from illegal Russian meddling.

In the end, there can be no innocent motive for, or innocent effect from, an attempt to halt a duly constituted investigation operating under proper constitutional safeguards. This is so even if a president's political opponents are misusing the existence of the investigation for their own illegitimate purposes.

If William Barr really thinks otherwise, he should not be attorney general.