At a March 31, 2016, meeting between Mr. Trump and his foreign policy team, Mr. Papadopoulos introduced himself and said “that he had connections that could help arrange a meeting between then-candidate Trump and President Putin,” according to court records.

“He went into the pitch right away,” said J. D. Gordon, a campaign adviser who attended the meeting. “He said he had a friend in London, the Russian ambassador, who could help set up a meeting with Putin.”

Mr. Trump listened with interest. Mr. Sessions vehemently opposed the idea, Mr. Gordon recalled. “And he said that no one should talk about it because it might leak,” he said.

Several of Mr. Trump’s campaign advisers attended the March 2016 meeting, and at least two of those advisers are now in the White House: Hope Hicks, the communications director, and Stephen Miller, a senior policy adviser.

After Mr. Trump was sworn in, he could not escape questions about Russia. At a Feb. 16, 2017, White House news conference, a reporter asked Mr. Trump, “Can you say whether you are aware that anyone who advised your campaign had contacts with Russia during the course of the election?”

“No,” Mr. Trump said. “Nobody that I know of. Nobody.”