In an unsolicited memorandum sent to top Justice Department officials this past June, William Barr — who served as attorney general under President George H.W. Bush and is now President Trump’s nominee for that post again — argued that the special counsel, Robert Mueller, should not be allowed to demand answers from the president about possible obstruction of justice. Mr. Barr’s memo, which became public this week, seriously damages his credibility and raises questions about his fitness for the Justice Department’s top position.

At the outset, Mr. Barr acknowledges that he is “in the dark about many facts” that bear on the special counsel’s inquiry. He also claims that Mr. Mueller — who has never said whether he believes that the president obstructed justice — “is proposing an unprecedented expansion of obstruction laws.” He then says that “Mueller’s theory should be rejected,” again without knowing what Mr. Mueller’s theory is.

To his credit, Mr. Barr does not toe the line taken by the president’s former personal attorney, John Dowd, who claimed in a confidential memo to the special counsel in January that “by virtue of his position as chief law enforcement officer,” the president cannot be liable for obstruction. “Obviously,” Mr. Barr acknowledges, “the President and any other official can commit obstruction” by “sabotaging a proceeding’s truth-finding function” through evidence destruction or witness tampering.

Instead, Mr. Barr offers a more narrow theory that the president can obstruct justice only by “evidence impairment,” which seems to mean shredding documents or threatening witnesses and the like. Firing the F.B.I. director for failing to act on the same evidence is apparently O.K. But this argument does not bear scrutiny.

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