In 1938 Stewart entered the race for the balance of the unexpired term of the late Senator Nathan L. Bachman, who had died in office. In the August Democratic primary he defeated labor union leader George L. Berry, who had been appointed to the seat upon Bachman's death by Governor Gordon Browning, and was elected Senator on November 8. Eligible to begin serving immediately, he instead waited until the expiry of his term as district attorney on January 16, 1939 to take his Senate seat.