Cooper served as the circuit judge of Kentucky's 28th judicial district until 1946, when he resigned to run for the Senate seat left by Albert Benjamin "Happy" Chandler. During this period, he frequently voted against his party so much so that colleague [[Robert A. Taft]] to ask whether he was a Republican or a [[Democratic Party|Democrat]].<ref>[https://www.nytimes.com/1991/02/23/obituaries/john-sherman-cooper-dies-at-89-longtime-senator-from-kentucky.html John Sherman Cooper Dies at 89; Longtime Senator From Kentucky]</ref>
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Cooper served as the circuit judge of Kentucky's 28th judicial district until 1946, when he resigned to run for the Senate seat vacated by [[A. B. "Happy" Chandler]], Sr. During this period, he frequently voted against his party so much so that colleague [[Robert A. Taft]] to ask whether he was a Republican or a [[Democratic Party|Democrat]].<ref>[https://www.nytimes.com/1991/02/23/obituaries/john-sherman-cooper-dies-at-89-longtime-senator-from-kentucky.html John Sherman Cooper Dies at 89; Longtime Senator From Kentucky]</ref>