Birch Evans Bayh Jr. (; January 22, 1928 – March 14, 2019) was an American politician who served as U.S. Senator from Indiana from 1963 to 1981. He was first elected to office in 1954, when he won election to the Indiana House of Representatives; in 1958, he was elected Speaker, the youngest person to hold that office in the state's history. In 1962, he ran for the U.S. Senate, narrowly defeating incumbent Republican Homer E. Capehart. Shortly after entering the Senate, he became Chairman of the Subcommittee on Constitutional Amendments, and in that role authored two constitutional amendments: the twenty-fifth—which establishes procedures for an orderly transition of power in the case of the death, disability, or resignation of the President of the United States—and the twenty-sixth, which lowered the voting age to 18 throughout the United States. He is the only non-Founding Father to have authored two constitutional amendments. Bayh also led unsuccessful efforts to ratify the Equal Rights Amendment and eliminate the Electoral College.
Bayh authored Title IX of the Higher Education Act of 1965, which bans gender discrimination in higher education institutions that receive federal funding. He also authored the Juvenile Justice and Delinquency Prevention Act, and co-authored the Bayh–Dole Act, which deals with intellectual property that arises from federal-government-funded research. He led the Senate opposition to Clement Haynsworth and G. Harrold Carswell, two of Richard Nixon's unsuccessful Supreme Court nominees. Bayh intended to seek the Democratic presidential nomination in 1972, but declined to run after his wife was diagnosed with cancer. He sought the Democratic presidential nomination in 1976, but dropped out of the campaign after disappointing finishes in the first set of primaries and caucuses.
Bayh won re-election in 1968 and 1974, but lost his 1980 bid for a fourth term to Dan Quayle. After leaving the Senate, he remained active in the political and legal world. His son, Evan Bayh, served as the 46th Governor of Indiana and held his father's former U.S. Senate seat from 1999 to 2011.
He is also known as Birch Evans Bayh Jr. and Birch Evans Bayh.