There was a class D league called the Wisconsin State League which began in 1905. In 1908 it changed its name to the Wisconsin-Illinois League. Another Wisconsin State League began in 1940, shut down for World War II from 1943 through 1945, then operated from 1946 through 1953. This was also a class D league, but with the majority of teams being affiliated with the major leagues, quality was likely higher than in many class D leagues. Also, the second incarnation was a very stable lineup, with only one franchise shift (La Crosse before the war, Wausau after). 5 of the 6 teams in 1940 made it through to 1953, and both teams added in 1941 making it through 1953 also.
This article is based on the "Wisconsin State League" article at Baseball-Reference.com Bullpen. The Bullpen is a wiki, and its content is available under the GNU Free Documentation License.