U.S. Attorney General Jeff Sessions delivers remarks about defending national security, at the U.S. Attorney's Office for the Southern District of New York, Thursday, Nov. 2, 2017. (Richard Drew/AP)

Senate Democrats have demanded that Attorney General Jeff Sessions explain why he did not disclose a March 2016 gathering with then-candidate Donald Trump and members of his campaign team at which an adviser offered to set up a meeting between Trump and Russian President Vladi­mir Putin.

Sessions's participation in the gathering was detailed in court documents released Monday by special counsel Robert S. Mueller III. The adviser who offered to set up the meeting was George Papadopoulos, who has pleaded guilty to lying to the FBI, according to the documents.

Sessions had not previously disclosed the meeting, despite being asked over multiple appearances on Capitol Hill whether he or anyone on the campaign ever discussed meeting with Russians.

"This is another example in an alarming pattern in which you, the nation's top law enforcement officer, apparently failed to tell the truth, under oath, about the Trump team's contacts with agents of Russia," Senate Judiciary Committee member Al Franken (D-Minn.) wrote Thursday in a letter to Sessions.

Senate Republicans have not rushed to Sessions's defense but were less ruffled about his participation in the Papadopoulos meeting. "If anybody's got any concerns about his meeting, write him a letter and see what he says," said Sen. Lindsey O. Graham (R-S.C.).

A Sessions spokeswoman declined to comment this week on the March 31 meeting with Trump and campaign advisers, including Papadopoulos. But on Thursday, a person familiar with Sessions's recollections said that "people who remember the conversation" believed that Papadopoulos was proposing an idea of using his Russian contacts to try to set up a meeting between Trump and Putin, "which was immediately rejected by then-Senator Sessions."

"Papadopoulos was some ­29-year-old that nobody had ever heard of and who struck people in the room as someone who didn't have a lot of credibility," the person said. "As far as Sessions seemed to be concerned, when he shut down this idea of Papadopoulos engaging with Russia, that was the end of it and he moved the meeting along to other issues."

"The March 31 comments by this Papadopoulos person did not leave a lasting impression," the person said. "The attorney general immediately dismissed it and moved on."

J.D. Gordon, a former Pentagon spokesman and Trump adviser who attended the meeting, also recalled that Sessions "shut him down" when Papadopoulos announced that he could introduce Trump to Putin.

"It was a bad idea and the Senator didn't want people to speak about it again," he said.

The Washington Post also reported earlier this week that Papadopoulos sat next to Sessions during a dinner at the Capitol Hill Club for campaign advisers weeks before the Republican National Convention, according to another person who attended.

[For ‘low level volunteer,’ Papadopoulos sought high profile as Trump adviser]

At the time of that article, a spokeswoman for Sessions declined to comment. But on Thursday, the person familiar with Sessions's recollections said, "the attorney general has no clear recollection of this person. He may have been at future Trump events that Sessions attended — including a dinner at the Capitol Hill Club with some members of the foreign policy committee over the summer — but Sessions doesn't recall any further interactions with him, including any phone calls or emails."

Gordon said that he did not fault Sessions for not recalling things that Papadopoulos said in private settings, noting that thousands of people approached the campaign, "including many, many true volunteers."

This is not the first time lawmakers have accused Sessions of withholding information from congressional testimony about interactions with or about Russians while working with the Trump campaign.

During his January confirmation hearing to become attorney general, Sessions told the Judiciary Committee that he had had no communication with Russian officials, only to have The Post reveal in March that he had spoken twice with the Russian ambassador to the United States, Sergey Kislyak.

[Sessions met with Russian envoy twice last year, encounters he later did not disclose]

Following the Post report, Sessions recused himself from the Justice Department's Russia probes — an action that infuriated Trump.

Last month, Sessions told the same committee he was not aware of any Trump surrogates having contacts with Russian officials. Democrats on the committee, including Franken, are now arguing that statement may have been false, too.

"He needs to clarify his testimony before our committee," said Judiciary Committee member Sen. Richard Blumenthal (D-Conn.). "There were questions that elicited exactly the kind of information that the Papadopoulos plea agreement made."

"The attorney general needs to explain to our committee what are the obvious implications of his presence at that meeting, and how he can square that with his testimony to our committee," said Sen. Christopher A. Coons (D-Del.), another member of the Judiciary Committee.

The person familiar with Sessions's interactions with Papadopoulos said that Sessions had been "entirely truthful and consistent on this matter" and that "he had no knowledge of any conversations by anyone connected to the Trump campaign with any Russian or any foreign officials concerning any type of interference with any campaign or election in the United States."

Democrats on the Senate Intelligence Committee, which is also looking into allegations of collusion and Russian meddling in the 2016 presidential election, also want a reckoning from Sessions.

"I think there are many more questions for Jeff Sessions," said Sen. Kamala D. Harris (D-Calif.), a member of the panel. "He should be honest and disclose all truths, especially when he is under oath or testifying before the United States Congress."

Rosalind S. Helderman contributed to this report.

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