Incumbent senator James J. Couzens lost the Republican primary and thus was not nominated for a third term. Couzens ultimately died on October 22, 1936. After the election, Prentiss Brown was appointed to fill the remainder of Couzen's term. As a result, Brown's service in the Senate is dated from November 19, 1936, not January 3, 1937.
Source for Popular Vote data: Michigan Secretary of State. Michigan Official Directory and Legislative Manual 1937-1938, Michigan Secretary of State (https://babel.hathitrust.org/cgi/pt?id=uc1.b4049842&view=1up&seq=287) (accessed 18 Sep 2023)