Wis. Stat. § 6.93: Challenging the absent elector.

The vote of any absent elector may be challenged for cause and the inspectors of election shall have all the power and authority given them to hear and determine the legality of the ballot the same as if the ballot had been voted in person. In municipalities where absentee ballots are canvassed under s. 7.52, the vote of an absentee elector may be challenged as provided in s. 7.52 (5).