Robert Kennedy's assassination in Los Angeles on June 5, 1968, was a more devastating blow to O'Donnell than the assassination of President Kennedy five years earlier. He soon joined, as did many others in Kennedy's campaign, Hubert Humphrey's presidential campaign, serving as campaign manager. In 1970, he made another attempt to win the Democratic nomination for governor, but finished fourth in a field of four Democrats, with just nine percent of the vote.