A twist of fate — a vacancy, and then an appointment to fill it — had propelled Mr. Mondale into state politics. Now came another that would send him to Washington. When Johnson selected Mr. Humphrey as his running mate, Mr. Mondale was chosen to fill Mr. Humphrey’s Senate seat. He was sworn in by Mr. Humphrey at the Bethesda Naval Medical Center, where Mr. Mondale had had an emergency appendectomy. He was later elected twice to the Senate with no difficulty.

In the Senate, Mr. Mondale lined up in favor of Johnson’s Great Society legislation, including the Voting Rights Act of 1965, and worked to enact fair housing laws against powerful opposition. He pressed for programs in education, child care, health care, jobs, desegregation and consumer protection.

One of his proudest legislative achievements, he said, was his leadership role in making it easier for the Senate to cut off a filibuster with 60 votes, under a rule change, rather than a two-thirds vote, as was previously required. One of his biggest regrets, he said, was his delay, until 1969, in turning against the Vietnam War.

By the 1970s Mr. Mondale’s name was on lists of possible candidates for national office. Dutifully, he wrote a campaign book, “The Accountability of Power: Toward a Responsible Presidency” (1975), in which he criticized the “imperial presidency” of Richard M. Nixon, and then joined the race for the 1976 presidential nomination.

The campaign went nowhere. “I remember that after a year I was running six points behind ‘Don’t Know,’” Mr. Mondale said in the 2010 interview. He ended the bid early, in 1974. In withdrawing, he said he lacked an “overwhelming desire to be president.” The comment would come to haunt him.

No. 2 With a Say

The Democratic victor, Mr. Carter, a conservative Southerner, was looking for a liberal running mate from the North who could help him pick up support in the industrial states. Mr. Mondale was at the top of everybody’s list, but he had mixed feelings until he got an agreement from the nominee that he would have a full-fledged policy role, expanded from the largely ceremonial functions assigned to most vice presidents.

Mr. Mondale’s chief of staff, Richard Moe, said Mr. Humphrey had been equally persuasive. “‘Fritz,’ he said, ‘if you have a chance to be vice president, you should take it,’” Mr. Moe recalled.