Mike Mansfield, 98, a self-effacing and down-to-earth Montana Democrat who retired in 1977 after 24 years in the U.S. Senate, 16 of them as majority leader, and who served as ambassador to Japan from 1977 to 1988, died yesterday at Walter Reed Army Medical Center. He had congestive heart failure.

Mansfield served as ambassador and majority leader during important periods -- and held each job longer than any of his predecessors.

He was Senate majority leader at a time of mounting congressional opposition to the war in Vietnam and the authority of the president to conduct war, as well as the Watergate crisis and the resignation of President Richard M. Nixon.

And his ambassadorship in Tokyo, one of the most sensitive posts in U.S. diplomacy, coincided with the years in which Japan established itself as one of the world's economic superpowers and built a staggering trade surplus with the United States.

In all, Mansfield's career in public life spanned five decades. It began in 1942 with his election to the House of Representatives, where he spent a decade before winning election to the Senate in 1952. He served as Democratic whip from 1957 to 1961, when he was chosen for the pivotal post of majority leader at the personal request of the newly elected President John F. Kennedy.

Mansfield intended to retire after deciding not to seek reelection to the Senate in 1976 but changed his mind at President Jimmy Carter's urging and accepted the post in Japan. He was retained as ambassador throughout the presidency of Ronald Reagan and resigned in 1988 at the age of 85.

President Bush noted Mansfield's death "with sadness," saying in a written statement, "He will be sorely missed, but his legacy of service to the United States will continue."

Senate Majority Leader Thomas A. Daschle (D-S.D.) recalled Mansfield as "my friend, my teacher and a leader whom I try to emulate," adding that "the Senate never had a finer leader." Senate Minority Leader Trent Lott (R-Miss.) remembered him as one of the Senate's giants.

Mansfield, a former professor of Far Eastern history at the University of Montana, brought a background of study and experience in Asian affairs to the embassy in Tokyo. As a young, man he had served in the Marine Corps in China, and he had been on Asian fact-finding missions for six U.S. presidents.

He was a junior member of the House of Representatives in 1945 when President Harry S. Truman sought his advice on terms of the Japanese surrender that ended World War II. Mansfield counseled the president to permit the Japanese to keep their emperor, a concession that proved to be vital to the success of the postwar peace agreement.

As ambassador he became an institution in Japan, venerated for his age, the status that he brought to the position and his sensitivity to Japanese customs and folkways. He brewed and served coffee to visitors at his embassy office, which impressed the decorum-conscious Japanese.

In 1987, when Japanese Crown Prince Akihito and the Crown Princess Michiko visited the United States, Mansfield took the unusual step of accompanying the imperial couple on their Japan Airlines jet across the Pacific and at every stop on their tour of the United States. When he returned to the United States after 11 years in Tokyo, the Japanese ambassador to this country said Mansfield "could have run for prime minister and won."

He preached a gospel of free trade on both shores of the Pacific and pestered Japanese bureaucrats to lower trade barriers to U.S. products while discouraging anti-Japanese sentiment in the United States. Although generally given credit for having helped ease the tensions generated by competing U.S. and Japanese economies and the trade imbalance, Mansfield was sometimes accused by U.S. business and labor interests of being overly protective of the Japanese.

As the Senate's majority leader, he had a low-key style of management that was often described as "effective without being oppressive." In contrast to his predecessor, Lyndon B. Johnson (D-Tex.), he never twisted arms, browbeat or threatened other senators to get his way, relying instead on respect, accommodation and persuasion. This sometimes drew complaints that his leadership was too passive. But Mansfield argued that each senator was entitled to conduct his business with minimal pressure from the leadership, and he was reluctant to force his convictions on others.

Once, when a newspaper editorial called his leadership a "tragic mistake," Mansfield declared in a speech on the Senate floor, "if that means that I am neither a circus ringmaster, the master of ceremonies of a Senate nightclub, a tamer of Senate lions, or a wheeler and a dealer, then I must accept the title."

He delegated authority and encouraged the Senate's committee chairmen to floor-manage their own bills, reasoning that because these were the men who knew most about their content, they were best equipped to guide them to enactment. He also encouraged younger senators to speak their minds, and he helped move them into positions of authority, breaking with a long-standing Senate tradition that new members should remain silent and defer to their seniors for four or five years.

In the process, he democratized the Senate. It came to be less dominated by a small and exclusive inner club of older men. It was said of Mansfield that he changed the attitude and character of the institution as much as any leader in its history.

His most significant Senate achievements, Mansfield said on leaving Capitol Hill, were initiating the Watergate Committee whose investigations led to President Nixon's resignation and his roles in winning passage of extending voting rights to 18-year-olds and in creating the committee that investigated abuses by the CIA. A foe of the Vietnam War, he regretted that he "never was able to stop it or slow it down."

He lacked the eloquence of such Senate contemporaries as the late Senate Republican leader Everett McKinley Dirksen (Ill.), and he could never match the nimble wit of Hugh Scott (Pa.), who succeeded Dirksen as Republican leader in 1969.

Mansfield's most frequent answers to questions from the media were "yep," "nope," "maybe" and "don't know," which did not endear him to the producers of television news talk programs.

But he was one of the most genuinely liked and respected men on Capitol Hill. Scott once called him "the most decent man I've met in public life." Dirksen once said he "would go anywhere to campaign for Republicans, even to the moon, but please, not Montana."

A large bureaucracy made Mansfield uncomfortable, so he kept his office staff small and employed no press secretary. He did use the chauffeur-driven limousine that went with the job of majority leader, and he often drew chuckles when he used the car to visit a free public golf course or a cut-rate clothing store where it was sometimes said that the hue of the suits he purchased would have depressed a mortician.

It was a paradox of Mr. Mansfield's life that he rose to a position of leadership in a body of ambitious and hard-driving men without ever appearing overtly ambitious or hard-driving. Once, when advised to dismiss a campaign aide whose work was unsatisfactory, he replied that the man had once done him a favor and that winning the election was not worth hurting his feelings.

He tried to schedule Senate business so that senators could get home for dinner on time, and he was known to keep Cabinet officers waiting while he visited with his constituents. All mail from his constituents was answered, and he read and signed every letter himself.

Michael Joseph Mansfield was born March 16, 1903, in New York, the son of Irish immigrant parents. His mother died when he was 3 years old, and he was sent to live with an aunt and uncle in Great Falls, Mont., where he worked in a family grocery store.

He dropped out of school before finishing the eighth grade. Shortly before his 15th birthday, he lied about his age and enlisted in the Navy. He served during World War I and crossed the Atlantic seven times on a military transport before the Navy discovered he was underage and discharged him.

Later, he served a year in the Army in California. In 1920, he joined the Marine Corps and during the next two years served in the Philippines, China and Siberia.

Returning to Montana in 1922, Mansfield took a job as a "mucker," shoveling rocks and dirt half a mile under the ground in a copper mine near Butte. Five years later, while still working in the mines, he enrolled as a special student at the Montana School of Mines. While there, he met his future wife, Maureen Hayes, a Butte schoolteacher who persuaded him to complete his high school education by taking correspondence courses.

In 1930, he left the mines and enrolled at the University of Montana, where he graduated three years later. He was awarded a position as a graduate assistant and the following year received a master's degree in history, then joined the faculty as an instructor. During the summers of 1936 and 1937, he did additional graduate study at the University of California at Berkeley and was subsequently appointed a full professor of Far Eastern and Latin American history and political science at Montana.

But he also wanted to try his hand at politics. "There's a little bit of political blood in all the Irish," Mansfield often said. In 1940, he made his first bid for public office, finishing last in a three-way race for the Democratic nomination for the House of Representatives from the congressional district that included the mining region in the western part of the state.

Two years later, he ran again for the same seat, winning this time in an election to fill the seat being vacated by the retirement of Republican Rep. Jeannette Rankin.

He was assigned immediately to the Foreign Affairs Committee, then in 1944 was sent on a confidential fact-finding mission to China by President Franklin D. Roosevelt. Returning in January 1945, Mansfield reported to the president that he found China a deeply divided and demoralized nation. He would later observe that the collapse of Sino-American relations following the Communist takeover on the Mainland was one of the great failures of U.S. foreign policy.

As a member of the House, Mansfield backed the extension of price controls, the immigration of persons displaced by World War II to the United States, increases in the minimum wage, economic aid to Greece and Turkey and the Marshall Plan for the economic rehabilitation of postwar Europe. He opposed the peacetime draft, the Taft-Hartley Act and poll taxes.

In 1952, he challenged Montana's incumbent Republican Sen. Zales N. Ecton. He narrowly won in what turned out to be a heated campaign that included visits to Montana by Wisconsin's Communist-baiting Republican Sen. Joseph R. McCarthy, who accused Mansfield of "Communist-coddling practices."

The new senator was assigned a seat on the Foreign Relations Committee, which in 1953 sent him on a fact-finding mission to Indochina, where the war between the French colonialists and the Viet Minh was in its final stages. The following year, he was the Senate's Democratic Representative at the Manila Conference that established the Southeast Asia Treaty Organization.

Mansfield was also assigned on the Senate's Appropriations Committee and his party's Policy and Steering Committee. He moved quickly into a leadership role. In 1957, he became whip, his party's assistant Senate leader, then moved up to majority leader when Johnson became vice president in 1961.

Although he had supported Johnson over Kennedy for the Democratic presidential nomination in 1960, Mansfield had a close and warm relationship with Kennedy during his presidency. After Kennedy's assassination in 1963, he delivered an emotional eulogy during a Capitol Rotunda memorial service, which was broadcast on national television.

Mansfield was one of the first prominent Americans to voice concern about developments in Vietnam, warning both the Eisenhower and Kennedy administrations that no government in Saigon could succeed unless it was based on social justice and popular with the people.

Initially, Mansfield supported the Johnson administration's plan for a negotiated settlement of the Vietnam War, but as U.S. involvement in the fighting escalated he became increasingly critical of U.S. policy and his relations with the Johnson White House were strained.

He introduced several legislative measures aimed at ending American participation in the hostilities and at curbing the power of the executive branch to conduct the war without the consent of Congress. Later, he would also support unilateral troop reductions in Europe and South Korea, arguing that having military forces on the scene increased the likelihood that they would be used.

Despite his strong disagreements with White House policy, Mansfield took pains to maintain civil relations with the presidents during his years as majority leader. His relations with Nixon were cordial although not close, but they did have breakfast once a month during Nixon's presidency.

His appointment as ambassador to Japan, just months after his retirement from the Senate, was widely greeted in Japan as an indication that President Carter recognized the critical importance of Japan to the United States and in the world. In the Japanese media, Mansfield became known as "omono taishi," or "big-name ambassador."

He found the pace of life as ambassador less hectic than it had been in the Senate, but he was ready for a lifestyle where high officials in Japan would appreciate him as a wise elder with many beneficial Washington connections. In time it would be said of Mansfield that he had the best of both worlds, officially the U.S. ambassador to Japan, but also thought of by many Japanese as their ambassador to the United States.

On retiring as ambassador, Mansfield was awarded a Presidential Medal of Freedom, the nation's highest civilian award.

After his ambassadorship, he served for a time as a senior adviser on East Asian affairs to Goldman, Sachs & Co., the Wall Street investment banking firm.

Maureen Mansfield, whom he married in 1932, died in 2000. Survivors include a daughter, Anne Mansfield Marris of London; and a granddaughter.

Mike Mansfield, left, speaks with Walter F. Mondale shortly after Mondale succeeded him as U.S. ambassador to Japan in 1988.Mansfield listens to then-Secretary of State William Rogers during hearings on the Vietnam War on Feb. 21, 1973.Mansfield, right, talks with Mondale. Mansfield was highly respected by the Japanese during his 11-year tenure as U.S. ambassador to Japan.