WASHINGTON — There’s little doubt Richard Nixon was up to dirty tricks before his presidency ever began.

Documents released by the National Archives’ Nixon Presidential Library on Wednesday add weight to considerable, existing evidence that his 1968 presidential campaign tried to sabotage Vietnam peace talks before the U.S. election. The apparent purpose: to deny his Democratic challenger, Hubert Humphrey, a political advantage in the tight race to succeed Lyndon Johnson in the White House.

The documents contain a memo written to Nixon by his aide Tom Charles Huston in 1970, looking back at the so-called Chennault Affair, named for Anna Chennault, a Republican activist who had ties to South Vietnam leaders. Chennault was thought to have been a secret GOP emissary, conveying messages from the Nixon campaign to the South Vietnamese that they should resist peace talks with North Vietnam because they would get a better deal if they waited for Nixon to become president.

Johnson was incensed by what he regarded as Nixon’s meddling, as a candidate, in Vietnam War policy. At one point he called Nixon treasonous. Federal law prohibits private citizens — which Nixon then was — from interfering in U.S. diplomacy.

Nixon’s people saw the episode differently. They thought LBJ was calling a bombing halt and encouraging peace talks merely to give fellow Democrat Humphrey a boost in the election. And they thought he was using the heavy hand of government surveillance and intimidation to get to the bottom of what the Nixon campaign was doing with the South Vietnamese — tactics Nixon’s men would practice ruthlessly against real and perceived adversaries once in office.