Robert La Follette would resign on January 1, 1906 after being elected to the United States Senate. He was succeeded by Lieutenant Governor James Davidson, who served the remainder of the term.
Candidates' party affiliations obtained from the 1905 Blue Book. The Blue Book contains a misprint for La Follette in St. Croix County. https://search.library.wisc.edu/digital/ALVHJTC5U76EDH9D/full/A5WHUFZVNCZUBJ9B
Source for Popular Vote data: Wisconsin Board of State Canvassers, "Governor," Tabular Statement of the votes...at the General Election (Madison, 1904)