The scope of the ethics conversation was limited to the statements about the inquiry that Mr. Whitaker made before joining the department. In the letter — addressed to Senator Mitch McConnell of Kentucky, the Republican majority leader, and Senator Chuck Schumer of New York, the Democratic leader — the Justice Department noted that no previous attorney general or senior department official had stepped down from a matter “based upon statements made in the media.”

Separately, the Justice Department provided to the Senate Judiciary Committee late on Wednesday a package of Mr. Barr’s writings and speeches. The documents included a 19-page, apparently unsolicited memo Mr. Barr sent to senior department officials in June in which he sharply criticized Mr. Mueller’s focus on whether Mr. Trump had obstructed justice.

[Read Mr. Barr’s memo.]

Public speculation about an obstruction charge has focused on actions like Mr. Trump’s pressuring the F.B.I. director at the time, James B. Comey, to quash a criminal investigation into his first national security adviser, Michael T. Flynn, for lying to investigators about his conversations with the Russian ambassador; Mr. Trump’s firing of Mr. Comey; and the president’s seeming dangling of potential pardons at witnesses in Mr. Mueller’s inquiry.

While acknowledging in his memo that he was “in the dark about many facts,” Mr. Barr argued that the Justice Department must not accept the notion that a president can violate a statute that criminalizes obstruction of justice when he is exercising his constitutional authority in an otherwise lawful way — such as by firing a subordinate, pardoning someone or using his “complete authority to start or stop a law enforcement proceeding” — but with a corrupt motive.

“Mueller’s obstruction theory is fatally misconceived,” Mr. Barr wrote. “As I understand it, his theory is premised on a novel and legally insupportable reading of the law. Moreover, in my view, if credited by the department, it would have grave consequences far beyond the immediate confines of this case and would do lasting damage to the presidency and to the administration of law within the executive branch.”

Mr. Barr’s memo was first reported by The Wall Street Journal.

Several Democrats reacted to the memo and the recusal decision with alarm. Representative Jerrold Nadler of New York, who will soon become chairman of the House Judiciary Committee, suggested that Mr. Whitaker was more concerned with satisfying Mr. Trump than protecting the department’s integrity. He said he would grill Mr. Whitaker at an oversight hearing next month.