Gary Warren Hart (né Hartpence; born November 28, 1936) is an American politician, diplomat, and lawyer. In 1984, he ran for the Democratic presidential nomination, finishing as the runner-up to Walter Mondale; he ran again for the nomination in 1988, and was initially considered the front-runner, but eventually dropped out amid revelations of extramarital affairs. He represented Colorado in the United States Senate from 1975 to 1987.

Born in Ottawa, Kansas, Hart 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".

Hart sought the Democratic presidential nomination in 1984, narrowly losing the race to former Vice President Walter Mondale. Hart declined to seek re-election to the Senate in 1986 and sought the Democratic presidential nomination in 1988. He was widely viewed as the front-runner until reports surfaced of an extramarital affair, and Hart withdrew from the race in May 1987. He re-entered the race in December 1987 but withdrew from the race again after faring poorly in the early primaries. The nomination ultimately went to Michael Dukakis.

Hart returned to private practice after the 1988 election and served in a variety of public roles. He co-chaired the Hart-Rudman Task Force on Homeland Security, served on the Homeland Security Advisory Council, and was the United States Special Envoy for Northern Ireland. He earned a doctorate in politics from the University of Oxford and has written for outlets such as The Huffington Post. He has also written several books, including a biography of President James Monroe. Hart married Lee Ludwig in 1958, who died at age 85 on April 9, 2021. They had two children, John and Andrea Hart.

Hart was born in Ottawa, Kansas, the son of Nina (née Pritchard) and Carl Riley Hartpence, a farm equipment salesman. As a young man, he worked as a laborer on the railroad. He and his father changed their last name to "Hart" in 1961 because "Hart is a lot easier to remember than Hartpence." Raised in the Church of the Nazarene (which he ultimately left in 1968), he won a scholarship to the Church-affiliated Bethany Nazarene College (now Southern Nazarene University) in Bethany, Oklahoma, in 1954 and graduated with a B.A. in philosophy in 1958. He met his wife, Oletha "Lee" Ludwig, there, and they married in 1958. Initially intending to enter the Nazarene ministry, he received a B.D. from Yale Divinity School in 1961 before receiving an LL.B. from Yale Law School in 1964.

Hart became an attorney for the United States Department of Justice from 1964 to 1965 and was admitted to the Colorado and District of Columbia bars in 1965. He was special assistant to the solicitor of the United States Department of the Interior from 1965 to 1967. He then entered private law practice in Denver, Colorado, at the firm of Davis Graham & Stubbs.

Following the 1968 Democratic National Convention in Chicago, U.S. Senator George McGovern of South Dakota co-chaired a commission that revised the Democratic presidential nomination structure. The new structure weakened the influence of such old-style party bosses as Chicago Mayor Richard J. Daley, who were once able to hand-pick national convention delegates and dictate the way they voted. The new rules made caucuses a process in which relative newcomers could participate without paying dues to established party organizations.

In the 1972 primary elections, McGovern named Hart his national campaign director. Along with Rick Stearns, an expert on the new system, they decided on a strategy to focus on the 28 states holding caucuses instead of primary elections. They felt the nature of the caucuses made them easier and less costly to win if they targeted their efforts. While their primary election strategy proved successful in winning the nomination, McGovern went on to lose the 1972 presidential election in one of the most lopsided elections in U.S. history.