Mich. Comp. Laws § 408.875

Sec. 5. Subject to section 8, a governmental unit awarding a contract on or after the effective date of the amendatory act that added section 2 for the construction, repair, remodeling, or demolition of a facility and any construction manager acting on its behalf shall not, in any bid specifications, project agreements, or other controlling documents: (a) Require or prohibit a bidder, offeror, contractor, or subcontractor from entering into or adhering to an agreement with 1 or more labor organizations in regard to that project or a related construction project. (b) Otherwise discriminate against a bidder, offeror, contractor, or subcontractor for becoming or remaining or refusing to become or remain a signatory to, or for adhering or refusing to adhere to, an agreement with 1 or more labor organizations in regard to that project or a related construction project.