John Smith would die in office on August 17, 1947. John Sonderegger was appointed on August 19th to fill the vacancy. Sonderegger would subsequently resign on September 30, 1948 to become Commissioner of Insurance. Clyde Johnston was appointed on October 1st to fill the vacancy and served the remainder of the term.
Source for Popular Vote data: Wisconsin Board of State Canvassers, "State Treasurer," Statement of Board of State Canvassers for State Officers General Election - 1946 (Madison, 1946)