Johnson suspected that Republican Nixon, running against Hubert Humphrey, Johnson's vice president, was interfering in the Vietnam war to sway the election, a move he classified as political sabotage or more to his view, treason, the recordings say.
The tapes, released Thursday, had been kept secret until now.
Johnson and his aides, trying to arrange peace talks between North and South Vietnam on the eve of election, were told that Nixon allies had asked South Vietnam to avoid peace talks until after the election, the tapes reveal.
Johnson and his advisers, Humphrey included, kept their concerns secret at the time, the Austin American-Statesman said. Nixon defeated Humphrey by just 500,000 votes out of 73 million cast.
The LBJ Library made the conversations public with the release of 42 hours of recordings made from May 1968 until the Johnson family left the White House in January 1969.