Labor unions are asking a federal judge in Detroit to block part of Michigan's right-to-work law from taking effect in late March.
The lawsuit filed Monday is the second to challenge the law in recent weeks. It prohibits requiring workers to pay union dues or fees as a condition of employment.
Unions say the law can't apply to private-sector employees because it overreaches into any area controlled by the National Labor Relations Act. AFL-CIO lawyer Andrew Nickelhoff also says the law is an unconstitutional attempt to control workplace activities in federal areas such as dockyards.
An earlier suit challenges how the law was rushed through the Republican-controlled Legislature.
Supporters of right-to-work laws say they give employees greater freedom of choice. Opponents say they undermine the ability to unionize.