Mr. Hathaway, a Democrat in an independent-minded state, won an open seat in the House of Representatives in 1964. He served for eight years before challenging Smith, a four-term Republican, for her Senate seat.
The first woman to be elected to both houses of Congress, Smith had drawn national attention for her bold opposition to Sen. Joseph R. McCarthy (R-Wis.) during his anti-Communist crusades of the 1950s. She became known as the “conscience of the Senate.”
In 1972, she ran what was described as a lackluster reelection bid while Mr. Hathaway, a quarter-century her junior, mounted an energetic shoe-leather campaign. His victory was regarded as one of the most striking upsets of the election. Years later, he recalled his family’s reaction to the news. “Mother said, ‘You ought to be ashamed of yourself,’ ” the Bangor Daily News quoted him as saying.
After one term, in another nationally watched race, Mr. Hathaway lost his Senate seat to then-U.S. Rep. William S. Cohen (R-Maine), the future secretary of defense.
Mr. Hathaway came to Washington with a background in law and the experience of war, having served in the Army Air Forces during World War II. In 1944, he participated in a bombing mission over the oil refineries of Ploesti, Romania — a critical strategic asset for the Germans — and was forced to jump from the plane after enemy fire hit it.
Mr. Hathaway parachuted to the ground, according to an account published in the Journal of Commerce, and offered his watch to a farmer in exchange for assistance. The farmer betrayed him, however, and for months, Mr. Hathaway was held in a prisoner of war camp.
Three decades later, by then a senator, Mr. Hathaway received a letter from a young woman in Maine who wished to apply to the U.S. Military Academy at West Point. When the Army notified Mr. Hathaway that the academy could not accept women because it did not have appropriate restroom facilities, he was “infuriated,” he told the Sun Journal of Lewiston, Maine.
He introduced an amendment to a military-spending bill allowing women to be admitted to the academies. In 1975, President Gerald R. Ford signed a law putting that change into effect.
Among Mr. Hathaway’s other chief legislative concerns were labor issues.
Sen. Angus King (I-Maine), a former Hathaway aide, said in an interview that Mr. Hathaway helped shepherd the Employee Retirement Income Security Act of 1974, a law setting standards for private pensions.
As a House member, Mr. Hathaway was credited with helping win enactment of the landmark Occupational Safety and Health Act of 1970, which regulates workplace safety conditions.
Mr. Hathaway took pride, his family said, in taking sometimes unpopular stands, such as his support for turning over the Panama Canal to Panamanian control in the late 1970s and the settlement of controversial Indian land claims in his home state. The latter issue became a matter of intense debate in his campaign against Cohen.
“I think, by and large, people trust someone who doesn’t agree with them all the time,” Mr. Hathaway said during his hard-fought 1978 Senate campaign. He lost that November.
William Dodd Hathaway was born Feb. 21, 1924, the son of a railroad engineer, in Cambridge, Mass. He reportedly enlisted in the military at 17, shortly after the Japanese attack on Pearl Harbor.
Mr. Hathaway met his future wife at a military hospital in Texas, where she was a nurse and where he was sent to recover from his wartime wounds. His decorations included the Distinguished Flying Cross, the Purple Heart and the Air Medal, his family said.
Mr. Hathaway received a bachelor’s degree in philosophy in 1949 and a law degree in 1953, both from Harvard University. He moved to Maine to work in the law office of Frank M. Coffin, who would later become a congressman and federal appeals court judge.
Mr. Hathaway challenged an incumbent Republican House member in 1962 and lost narrowly. He became chairman of the Maine Democratic Party before heading to Washington after the next election.
As a junior House member, Mr. Hathaway supported the civil rights and anti-poverty programs of President Lyndon B. Johnson. In the Senate, he was among the few lawmakers who called for delaying Ford’s confirmation as vice president until President Richard M. Nixon’s impeachment matter was settled.
After his political career, Mr. Hathaway did lobbying work for the Washington firm of Patton, Boggs and Blow. In 1990, President George H.W. Bush appointed him to the Federal Maritime Commission, which regulates matters including shipping. He served on the commission, including several years as chairman, until his retirement in 1996.
His wife of 61 years, Mary Bird Hathaway, died in 2007.
Survivors include two children, Susan Hathaway Boydston of Cincinnati, and Fred Hathaway of the District; five grandchildren; and a great-grandson.
In the interview, King recalled Mr. Hathaway’s generosity to his Capitol Hill aides. While working for the senator, King underwent surgery after a diagnosis of melanoma. When he awoke, he saw Mr. Hathaway standing at the foot of his bed in green scrubs.
“I was 28 years old, and yet he left the Senate” to come to the hospital, King said. “I will never, ever forget that.”