Mr. Papadopoulos was named by Mr. Trump in March 2016 as one of five foreign policy advisers. While the president and his team now seek to minimize Mr. Papadopoulos’s importance, at the time Mr. Trump described him in flattering terms. “He’s an energy and oil consultant, excellent guy,” he told The Washington Post.

According to a statement of offense signed as part of his guilty plea, Mr. Papadopoulos admitted that he spent months last year cultivating contacts in an effort to arrange meetings between Mr. Trump’s campaign and Russian government officials.

Mr. Papadopoulos said a London-based professor with extensive Russian contacts introduced him to a woman described as “Putin’s niece” and told him the Russians had “dirt” on Mrs. Clinton based on “thousands of emails” of hers. (The woman was not actually related to President Vladimir V. Putin.)

The professor, identified on Monday by a Senate aide as Joseph Mifsud, told Mr. Papadopoulos about the emails in April 2016, three months before WikiLeaks released nearly 20,000 hacked Democratic emails. Mr. Papadopoulos kept senior campaign officials informed about his efforts and they encouraged him but made clear they wanted to keep some distance publicly. “It should be someone low level in the campaign so as not to send any signal,” a top campaign official wrote in an email at the time.

Mr. Papadopoulos was not charged with any crime for making those efforts but instead pleaded guilty to lying to F.B.I. agents about the matter. He was arrested secretly in late July and has been cooperating ever since with the team of the special counsel, Robert S. Mueller III.

“George was a low-level volunteer who might have attended a meeting of the foreign policy advisory team, the one meeting that took place but he was not a person who was involved with the day-to-day operations of the campaign, or a person who I recall interacting with on a regular basis at all,” Corey Lewandowski, who ran the campaign before Mr. Manafort, said on the “Today” show on NBC on Tuesday.

Mr. Lewandowski said he did not learn about the hacked Democratic emails until they became public. “To the best of my knowledge, absolutely not,” he said. “When I found out about that, I found out about it through public press reports.” He added that he has not spoken with the F.B.I. but would be “happy to do that unequivocally.”