Wisconsin Statutes Employment, Compensation and Mining (Ch. 101 to 109) § 103.52. “Yellow-dog” contracts

(1) Every undertaking or promise made after July 1, 1931, whether written or oral, express or implied, between any employee or prospective employee and that person's employer, prospective employer or any other individual, firm, company, association or corporation is declared to be against public policy if either party to the undertaking or promise undertakes or promises any of the following:

(2) No undertaking or promise described in sub. (1) shall afford any basis for the granting of legal or equitable relief by any court against a party to the undertaking or promise, or against any other person, who may advise, urge or induce, without fraud, violence or threat of fraud or violence either party to the undertaking or promise to act in disregard of the undertaking or promise.

(3) This section in its entirety is supplemental to and of s. 103.46(1).