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.