Wis. Stat. § 111.04: Rights of employees.

(1) Employees shall have the right of self-organization and the right to form, join or assist labor organizations, to bargain collectively through representatives of their own choosing, and to engage in lawful, concerted activities for the purpose of collective bargaining or other mutual aid or protection.

(2) Employees shall have the right to refrain from self-organization; forming, joining, or assisting labor organizations; bargaining collectively through representatives; or engaging in activities for the purpose of collective bargaining or other mutual aid or protection.

(3)

(a) No person may require, as a condition of obtaining or continuing employment, an individual to do any of the following:

1. Refrain or resign from membership in, voluntary affiliation with, or voluntary financial support of a labor organization.

2. Become or remain a member of a labor organization.

3. Pay any dues, fees, assessments, or other charges or expenses of any kind or amount, or provide anything of value, to a labor organization.

4. Pay to any 3rd party an amount that is in place of, equivalent to, or any portion of dues, fees, assessments, or other charges or expenses required of members of, or employees represented by, a labor organization.

(b) This subsection applies to the extent permitted under federal law. If a provision of a contract violates this subsection, that provision is void.