AUSTIN — In the last months of his administration, President Lyndon B. Johnson suggested that associates of Richard M. Nixon were trying to persuade the South Vietnamese government not to join the peace talks until after the 1968 election, recordings of telephone conversations released Thursday show.

Progress on peace in Vietnam before the election might have given Hubert H. Humphrey, the Democratic presidential nominee and Johnson’s vice president, support among voters.

Accusations of Nixon’s influence in the peace conference have been reported before, but the tapes provide a look at how Johnson handled the issue, said Bruce Buchanan, a government professor and an expert on the presidency at the University of Texas, Austin.

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.