Born in Ottawa, Kansas, he pursued a legal career in Denver, Colorado, after graduating from Yale Law School. He managed Senator George McGovern's successful campaign for the 1972 Democratic presidential nomination and McGovern's unsuccessful general election campaign against President Richard Nixon. Hart defeated incumbent Republican Senator Peter Dominick in Colorado's 1974 Senate election. In the Senate, he served on the Church Committee and led the Senate investigation regarding the Three Mile Island accident. After narrowly winning re-election in 1980, he sponsored the Semiconductor Chip Protection Act of 1984, becoming known as an "Atari Democrat".