Richard M. Nixon told an aide that they should find a way to secretly “monkey wrench” peace talks in Vietnam in the waning days of the 1968 campaign for fear that progress toward ending the war would hurt his chances for the presidency, according to newly discovered notes.

In a telephone conversation with H. R. Haldeman, who would go on to become White House chief of staff, Nixon gave instructions that a friendly intermediary should keep “working on” South Vietnamese leaders to persuade them not to agree to a deal before the election, according to the notes, taken by Mr. Haldeman.

The Nixon campaign’s clandestine effort to thwart President Lyndon B. Johnson’s peace initiative that fall has long been a source of controversy and scholarship. Ample evidence has emerged documenting the involvement of Nixon’s campaign. But Mr. Haldeman’s notes appear to confirm longstanding suspicions that Nixon himself was directly involved, despite his later denials.

“There’s really no doubt this was a step beyond the normal political jockeying, to interfere in an active peace negotiation given the stakes with all the lives,” said John A. Farrell, who discovered the notes at the Richard Nixon Presidential Library for his forthcoming biography, “Richard Nixon: The Life,” to be published in March by Doubleday. “Potentially, this is worse than anything he did in Watergate.”