Michael Joseph Mansfield (March 16, 1903 – October 5, 2001) was an American Democratic Party politician and diplomat who represented Montana in the United States House of Representatives from 1943 to 1953 and United States Senate from 1953 to 1977. As the leader of the Senate Democratic Caucus from 1961 to 1977, Mansfield shepherded Great Society programs through the Senate; his tenure of exactly sixteen years was the longest of any party leader in Senate history, until the record was broken by Mitch McConnell in 2023. Wikipedia
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