His death was announced in a statement by his family. He had been moved to hospice care in recent days after being treated for several health problems in the last year. He had a home in Mitchell, S.D., where he had spent his formative years.

In a statement, President Obama called Mr. McGovern "a champion for peace" who was a "statesman of great conscience and conviction."

To the liberal Democratic faithful, Mr. McGovern remained a standard-bearer well into his old age, writing and lecturing even as his name was routinely invoked by conservatives as synonymous with what they considered the failures of liberal politics.

He never retreated from those ideals, however, insisting on a strong, "progressive" federal government to protect the vulnerable and expand economic opportunity, while asserting that history would prove him correct in his opposing not only what he called "the tragically mistaken American war in Vietnam" but also the American invasions of Iraq and Afghanistan.

A slender, soft-spoken minister's son newly elected to Congress — his father was a Republican — Mr. McGovern went to Washington as a 34-year-old former college history teacher and decorated bomber pilot in World War II. He thought of himself as a son of the prairie as well, with a fittingly flat, somewhat nasal voice and a brand of politics traceable to the Midwestern progressivism of the late 19th century.

Elected to the Senate in 1962, Mr. McGovern left no special mark in his three terms, but he voted consistently in favor of civil rights and antipoverty bills, was instrumental in developing and expanding food stamp and nutrition programs, and helped lead opposition to the Vietnam War in the Senate.

The war was the cause he took into the 1972 election, one of the most lopsided in American history. Mr. McGovern carried only Massachusetts and the District of Columbia and won just 17 electoral votes to Nixon's 520.

The campaign was the backdrop to the burglary at the Democratic Party headquarters in the Watergate Hotel in Washington and to the Nixon organization's shady fund-raising practices and sabotage operations, later known as "dirty tricks," which were not disclosed until after the election.

The Republicans portrayed Mr. McGovern as a cowardly left-winger, a threat to the military and the free-market economy and someone outside the mainstream of American thought. Whether those charges were fair or not, Mr. McGovern never lived down the image of a liberal loser, and many Democrats long accused him of leading the party astray.

Mr. McGovern resented that characterization mightily. "I always thought of myself as a good old South Dakota boy who grew up here on the prairie," he said in an interview for this obituary in 2005 in his home in Mitchell. "My dad was a Methodist minister. I went off to war. I have been married to the same woman forever. I'm what a normal, healthy, ideal American should be like.

"But we probably didn't work enough on cultivating that image," he added, referring to his presidential campaign organization. "We were more interested in ending the war in Vietnam and getting people out of poverty and being fair to women and minorities and saving the environment.

"It was an issue-oriented campaign, and we should have paid more attention to image."

The 1972 Nomination

Mr. McGovern was 49 years old and in his second Senate term when he won the 1972 Democratic nomination, outdistancing a dozen or so other aspirants, including Senator Edmund S. Muskie of Maine, the early front-runner; former Vice President Hubert H. Humphrey, the nominee in 1968; and Gov. George C. Wallace of Alabama, a populist with a segregationist past who was gravely wounded in an assassination attempt in Maryland during the primaries.

David E. Rosenbaum, a Washington correspondent for The New York Times, died in 2006. William McDonald contributed reporting.

This article has been revised to reflect the following correction:

Correction: October 21, 2012

An earlier version of this obituary contained these errors: Mr. McGovern was 49, not 50, when he won the Democratic Party's presidential nomination in 1972. His wife's maiden name was Stegeberg, not Stageberg. The navigator when Mr. McGovern crash-landed his bomber did not die on that flight; he was killed on another mission, when Mr. McGovern was not flying the plane.