Michigan Compiled Laws, Chapter 423. Labor Disputes and Employment Relations § 423.17a

Sec. 17a. It shall be unlawful for a labor organization or its agents to picket or cause to be picketed, or threaten to picket or cause to be picketed, any employer where the primary object thereof is forcing or requiring an employer to recognize or bargain with a labor organization as the representative of his employees, or forcing or requiring the employees of an employer to accept or select such labor organization as their collective bargaining representative.

unless the picketing labor organization is currently certified or lawfully recognized as the representative of such employees.