In 1980, Mr. Bayh was targeted by Republicans energized by Ronald Reagan’s presidential bid and was defeated by a brash young challenger, Rep. Dan Quayle, later vice president under George H.W. Bush. But the Bayh name remained resonant in Indiana, and his elder son, Evan, served as governor and U.S. senator.

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Birch Bayh, “just a shirttail lawyer from Shirkieville” in his words, was an unlikely avatar of constitutional reform when he arrived in Washington in 1963 after ousting a prominent three-term incumbent.

By chance, he landed on the Senate Judiciary Committee, although he was just three years out of law school and had more experience as a farmer than as a lawyer.

Then serendipity struck — twice. The constitutional amendment subcommittee’s chairman died, and no one wanted what seemed a ticket to obscurity. Mr. Bayh volunteered. President John F. Kennedy’s assassination three months later, in November 1963, elevated the job’s status dramatically.

Lyndon B. Johnson’s accession to the presidency was a stark reminder of a flaw in the succession process. There was no method to replace Johnson as vice president, and he had a history of heart disease. The two officials designated by statute as the first and second heirs — the speaker of the House and Senate president pro tem — were elderly and frail.

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The subcommittee became a vehicle to prominence. Chairman Bayh jumped aboard, becoming the main author and advocate of the 25th Amendment. Ratified in 1967 after protracted controversy, the amendment established clear procedures for appointing a vice president if a vacancy occurred. It also set rules for replacing the president should the incumbent become seriously disabled.

“A constitutional gap that has existed for two centuries has been filled,” Mr. Bayh said.

During the Watergate crisis, President Richard M. Nixon used the 25th Amendment in 1973 to name House Minority Leader Gerald R. Ford (R-Mich.) vice president. Ford succeeded Spiro T. Agnew, who resigned in disgrace after a federal investigation into allegations of bribery and extortion unrelated to Watergate. When Nixon resigned the next year, Ford chose Nelson Rockefeller as vice president.

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Mr. Bayh also wrote the 26th Amendment, adopted in 1971, setting the national voting age at 18. It settled an issue dating to World War II, when the slogan “old enough to fight, too young to vote” gained currency.

ERA and Title IX

Next Mr. Bayh co-wrote what would have been the 27th Amendment, the Equal Rights Amendment, prescribing equal treatment of women in all fields. Congress approved it in 1972. Sensing that the measure might sink because of opposition in state legislatures — ultimately, it did — Mr. Bayh produced Title IX of the 1972 Education Amendments. It banned gender discrimination in schools receiving federal support.

Title IX provoked controversy lasting decades, particularly the requirement that schools devote equal resources to male and female athletes. Notre Dame athletic director Edward W. “Moose” Krause, an Indiana icon, warned Mr. Bayh, “This thing is going to kill football.”

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Forty years after Title IX’s enactment, when Mr. Bayh was being honored by female professional basketball players, he recalled the argument he made in the 1970s: “In a country that prides itself on equality, we could not continue to deny 53 percent of the American people equal rights.”

Title IX had even an broader impact in classrooms and labs. In an interview when she was president of the University of Miami, Donna Shalala, Bill Clinton’s secretary of health and human services and now a congresswoman from Florida, said: “Title IX was a game changer. It created opportunities for women students, faculty, administrators. Without it, you wouldn’t see as many women studying law and medicine — or serving as university presidents.”

Feminism, Mr. Bayh acknowledged, was a taste he had acquired with the help of his first wife and political partner, Marvella Hern Bayh. “From time to time,” he reminisced in 2004, “she would remind me what it was like to be a woman in a man’s world. Without her, I would not have been in a leadership role” on women’s issues.

Early life

Birch Evans Bayh Jr. was born Jan. 22, 1928, in Terre Haute, Ind., close to Shirkieville, where forebears had farmed for generations. Birch Bayh Sr. was an athletic director who in 1935 moved the family to Montgomery County, Md., when he became director of physical education for the D.C. public school system.

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Mr. Bayh was 12 when his mother died, and he moved to his grandparents’ Shirkieville farm. He grew tomatoes that won a state ribbon and enrolled in the agriculture program at Purdue University in West Lafayette, Ind. After a two-year hiatus for Army service, he graduated in 1951 with a diverse résumé.

His senior classmates elected him president. He excelled in boxing and baseball. A strong debater, he represented Indiana at the American Farm Bureau’s national debate competition in Chicago.

Oklahoma’s gladiator, though only a freshman at Oklahoma State, was Marvella Hern. Their love-at-first-sight encounter did not distract her. She walked off with the national prize, plus his fraternity pin.

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They married in 1952, and the newlyweds ran his family’s farm. But he was restless and, in 1954, won a seat in the Indiana House of Representatives. Politics was so alien to his family, he joked, “my dad started thinking ‘where did I go wrong as a parent?’ ”

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Other Democrats had no such qualms. Swiftly, the articulate, handsome rural representative became minority leader and then speaker. While still responsible for the farm and legislative duties, he entered Indiana University law school. He graduated in 1960, joined a law firm in Terre Haute in 1961 and rented out the farm.

He had scarcely begun his new occupation when the Bayhs hatched a larger ambition: challenging Sen. Homer E. Capehart, a conservative Republican seeking a fourth term. The goal seemed grandiose. Indiana had voted overwhelmingly for Nixon in 1960, and Capehart was popular.

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But the Bayhs practiced retail politics relentlessly. “I’d rather shake hands than eat,” he liked to say. At one debate, the challenger rattled the incumbent, who advocated a military response to Communist Cuba, by accusing him of being a “warmonger.” Capehart seized Mr. Bayh by the lapels and exclaimed, “Don’t try to get away!”

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Reporters separated them before blows were struck. Mr. Bayh won by a margin of less than 1 percent. Time magazine opined that Capehart lost because “his image was that of a conservative who had just crept out of a cave.” The senator-elect said voters “are impressed by a fellow who’s out there working his tail off.”

Diligence remained the Bayh hallmark. He was active in drafting civil rights bills during the Kennedy and Johnson administrations even though such legislation was unpopular in Indiana.

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A delayed vote on a civil rights measure in 1964 almost killed him. The Bayhs accompanied Sen. Edward M. Kennedy to a Democratic event in Massachusetts, leaving hours later than scheduled. Their small plane crashed, landing in evening fog at a rural airport.

Two of the five people aboard died. The Bayhs suffered relatively minor injuries, but Kennedy’s back was broken. “Anybody alive up there?” Mr. Bayh called from the ground. “I’m alive,” Kennedy croaked.

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He was also immobile. Four months later, still in the hospital, Kennedy described to reporters how he locked his arms around his rescuer’s neck as Mr. Bayh, moving backward, dragged him out of the wreckage.

Mr. Bayh’s national profile grew in the late 1960s and early 1970s because of major battles over two Supreme Court nominees.

When Nixon nominated Clement F. Haynsworth Jr., chief judge of the U.S. Court of Appeals for the 4th Circuit, to a seat on the high court in 1969, a seemingly solid coalition of Republicans and Southern Democrats supported him. But union and civil rights leaders considered the conservative Haynsworth an enemy.

Mr. Bayh took the lead in rallying the opposition. With a few allies, he built a case against Hayns­worth, in part by casting doubt on his ethics. Haynsworth had participated in a case involving a company in which he owned stock. Writing in the New York Times in 1970, journalist Robert Sherrill observed that “Bayh is a master of the gentlemanly shiv.”

The Senate rejected Haynsworth and, in 1970, with Mr. Bayh again in the vanguard, voted down another conservative appellate court judge, G. Harrold Cars­well. Nixon accused Bayh and others of exceeding the Senate’s “advise and consent” authority.

The president, Mr. Bayh responded, is “wrong as a matter of constitutional law, wrong as a matter of history, and wrong as a matter of public policy.”

'Inspired' legislation

By the early 1970s, Mr. Bayh had made friends among groups with influence on Democratic politics: labor, feminists, the civil rights movement. Presidential ambition naturally followed, and he joined the field competing for the 1972 nomination before withdrawing after his wife received a diagnosis of breast cancer.

Marvella Bayh succumbed to a recurrence of cancer in 1979. Two years later, Mr. Bayh married Katherine “Kitty” Halpin, a director of news information for ABC News. In addition to his wife, survivors include a son from his first marriage, Evan Bayh; a son from his second marriage, Christopher Bayh; and four grandchildren.

During Marvella’s relapse, the Bayhs were frustrated that a promising treatment was unavailable because of a dispute over intellectual property rights. Medical and other innovations developed with government support, they discovered, sometimes remained in limbo because of procedures necessary to establish ownership.

Working with Sen. Robert J. Dole (R-Kan.), Mr. Bayh wrote a patent bill introduced in 1978 and enacted in 1980. It streamlined practices, expediting the availability of many scientific processes.

The Economist magazine called it “possibly the most inspired piece of legislation to be enacted in America over the past half century.”

It was his last legislative accomplishment. After his defeat in 1980, Mr. Bayh turned to the practice of law, ultimately settling into a partnership in the Washington office of Venable LLP. He never ran for office again, but he remained in the public arena. When Title IX cases reached the Supreme Court, he wrote amicus briefs defending his best-known legislation.

In 2008, at age 80, he campaigned throughout Indiana for Barack Obama, sometimes making five appearances a day. He told an Indianapolis Star reporter that his 1962 margin amounted to two votes per precinct. Hence his appeal to supporters:

“When it’s about over, and you’re so tired you can’t make another phone call, can’t take another step, get just two more votes for Birch.”

Obama carried Indiana in 2008 by less than 1 percent.