During a conversation with Senator Everett Dirksen of Illinois, Johnson, referring to people close to Nixon, and said, “This is treason.” Dirksen was the Republican leader in the Senate.

Image President-elect Richard M. Nixon and President Lyndon B. Johnson, meeting in the White House in December 1968. Credit... Charles Tasnadi/Associated Press

In a conversation in November 1968, Nixon assured Johnson that he supported the president’s efforts to arrange a peace conference in Paris. Johnson had cited news articles and private information he had been given that he said made him think Nixon’s associates were working against his efforts. ”I think what’s new here is the way Johnson characterizes it as ‘treason’ in his private conversations,” Professor Buchanan said. The 42 hours of telephone recordings released Thursday cover the period from May 1968 through January 1969, when Johnson left office.

In another recording, Johnson expresses his condolences to Senator Edward M. Kennedy after the assassination of Senator Robert F. Kennedy in June 1968.

“Ted, I know what a burden you bear, but your shoulders are broad and you’ve got lots of people who love you and who want to help you,” Johnson said.

Mr. Kennedy’s voice is barely audible, but he can be heard thanking Johnson and telling him “both my parents appreciate” his condolences.