Wis. Stat. § 6.325
No person may be disqualified as an elector unless the municipal clerk, board of election commissioners or a challenging elector under s. 6.48 demonstrates beyond a reasonable doubt that the person does not qualify as an elector or is not properly registered. If it appears that the challenged elector is registered at a residence in this state other than the one where the elector now resides, the municipal clerk or board of election commissioners shall, before permitting the elector to vote, require the elector to properly register and shall notify the municipal clerk or board of election commissioners at the former residence. The municipal clerk or board of election commissioners may require naturalized applicants to show their naturalization certificates.
Wis. Stat. § 6.325