For most of American history, vice presidential candidates were chosen for openly political reasons such as balancing a ticket or winning over a disgruntled faction. That strategy changed 40 years ago when Vice President Hubert Humphrey chose Sen. Edmund Muskie as his running mate. Muskie was, quite simply, the best man, and Humphrey understood that choosing a running mate based on that criteria was also good politics.

Humphrey had seen the vice presidency from virtually every angle when it came time for him to choose a running mate in 1968. He had been heartbroken when Adlai Stevenson had thrown the 1956 choice to the convention, a battle Humphrey knew he could not win. Lyndon B. Johnson had toyed with Humphrey before finally selecting him as his running mate in 1964 and then had discovered new ways to humiliate Humphrey during his service as vice president.

Humphrey had watched as presidential nominees selected their running mates for a variety of reasons. Some were chosen to help carry a crucial region of the country, like Johnson in 1960. Others were chosen to emphasize particular issues, considerations that influenced the selections of Richard M. Nixon in 1952 and Henry Cabot Lodge in 1960. Others owed their selection to their popularity with the party base, a factor that helped place Alben Barkley on the 1948 Democratic ticket.

Campaign skill sometimes was the key. Asked in 1964 why he had chosen Rep. William Miller, Sen. Barry Goldwater replied, "He drives Johnson nuts." Goldwater hoped Miller's skill at wielding the political hatchet would cause Johnson to lose his cool.

In 1964, Humphrey was the favorite of the party's liberal base. Still, Johnson was so far ahead that he could disregard political factors.

Humphrey enjoyed no such luxury four years later. Far from it. The war in Vietnam had divided the country and the Democratic Party. The convention in Chicago was disastrous — inside the hall, the party was split over Vietnam; outside the hall, television showed the ugly clashes between Chicago police and protesters. Humphrey was having difficulty emerging from Johnson's shadow and Johnson's actions made Humphrey's task all the more difficult.

Under these circumstances, Humphrey might have been forgiven for choosing a running mate with standard political considerations in mind. Muskie would seem to offer little by conventional measures. A Polish American and Catholic, he might have some appeal to ethnic Democrats tempted to support George Wallace's third-party campaign. Yet he was from Maine, a small state with only four electoral votes, and was little known.

Humphrey had told Muskie during the spring that, if he were free to select a running mate on the merits, Muskie would be his choice. Humphrey had watched Muskie for a decade in the Senate and had come to appreciate him as one of the most skillful legislators of his generation. Humphrey appreciated Muskie's willingness to listen to other views and his ability to persuade. And Humphrey recognized Muskie as a person of impeccable integrity. Muskie was the most presidential choice available to Humphrey.

Muskie quickly became a success on the campaign trail. He refused to pander to the preferences of his audiences. Instead, he spoke of basic American values — equal opportunity, pluralism, the democratic process — which he implored his listeners to embrace in deed as well as word.

Muskie soon emerged as a campaign asset. He became the most compelling figure on any ticket, particularly when compared with his vice presidential rivals, Gov. Spiro T. Agnew and Gen. Curtis Lemay. His authenticity appealed to voters and media alike.

The Humphrey campaign sought to exploit Muskie's appeal. It spent precious dollars on commercials comparing Muskie and Agnew and displayed Muskie prominently on television.

In choosing a presidential running mate, Humphrey demonstrated his faith in the people. He bet that they would judge him more favorably if he chose someone well qualified to be president than if he chose based on the criteria that had been standard.

Humphrey narrowly lost the election, but Muskie's presence on the ticket helped him narrow the gap at the end. Humphrey's insight and Muskie's campaign helped establish a new threshold requirement for vice presidential candidates, which most later presidential candidates have embraced: They have to be presidential.

Joel K. Goldstein, author of "The Modern American Vice Presidency: The Transformation of a Political Institution," is working on a book about Edmund Muskie.