The 1940 United States Senate election in Wisconsin was held on November 5, 1940.
Incumbent Progressive U.S. Senator Robert La Follette Jr. was elected to a third term in office over Republican Fred H. Clausen and Democrat James E. Finnegan.
Progressive primary
Candidates
Robert M. La Follette Jr., incumbent Senator since 1925
Results
Republican primary
Candidates
Walter B. Chilsen, former State Assemblyman (1919–21) and editor of the Merrill Daily Herald Fred H. Clausen, farming equipment manufacturer Michael G. Eberlein Glenn Frank, former President of the University of Wisconsin-Madison John P. Koehler, Health Commissioner of Milwaukee William C. Maas Reuben W. Peterson, former State Assemblyman (1937–39)
Campaign
Frank died two days before the primary, on September 15, but remained on the ballot.
Results
Democratic primary
Candidates
William D. Carroll, former State Senator (1933–37) John Cudahy, former U.S. Ambassador to Poland, Luxembourg, Belgium, and the Irish Free State (write-in) James E. Finnegan, former Wisconsin Attorney General (1933–37) James J. Kerwin
Results
General election
Candidates
Fred H. Clausen, farming equipment manufacturer (Republican) James E. Finnegan, former Wisconsin Attorney General (Democratic) Ted Furman (Communist) Robert M. La Follette Jr., incumbent Senator since 1925 (Progressive) Adolf Wiggert (Socialist Labor)