Fortney Hillman "Pete" Stark (1931-2020) was a long-time United States Representative; he served in the US House of Representatives from 1973 to 2013.
He was the winner of the Secular Coalition for America's "Find an Atheist, Humanist, Freethinker Elected Official" contest to find the highest-ranking elected US official with nontheistic beliefs.
He called himself "a Unitarian who does not believe in a Supreme Being." [1]
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He did not call himself an atheist, but rather a nontheist. But his beliefs may fall within this site's definition of "atheist".
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Pete Stark - Wikipedia