Wisconsin Statutes Employment, Compensation and Mining (Ch. 101 to 109) § 103.46. Contracts; promises to withdraw from or not to join labor, employers' or cooperative organizations are void

(1) A contract or agreement of hiring or employment between any employer and any employee or prospective employee, in which either party to the contract or agreement undertakes or promises not to join, become or remain a member of any labor organization or of any organization of employers, or in which either party to the contract or agreement undertakes or promises to withdraw from the employment relation if the party joins, becomes or remains a member of any labor organization or of any organization of employers.

(2) A contract or agreement for the sale of agricultural, horticultural or dairy products between a producer of those products and a distributor or purchaser of those products, in which either party to the contract or agreement undertakes or promises not to join, become or remain a member of any cooperative association organized under ch. 185 or 193 or of any trade association of the producers, distributors or purchasers of those products.