Clark lost the Democratic primary for a second term to his predecessor, C. Ben Ross, who ran for United States Senate in 1936 against longtime incumbent William Borah. Ross lost the general election in 1938 to C. A. Bottolfsen; Clark left office on January 2, 1939, and returned to Idaho Falls and his private interests. He chose not to run in 1940 to reclaim the office, which was won by his younger brother Chase Clark (1883–1966). Chase was the father-in-law of Frank Church (1924–84), a four-term U.S. Senator (1957–81) and presidential candidate in 1976. In turn, Bottolfsen defeated Chase in 1942.