The Missouri Senate voted Thursday to become the 28th state to adopt a right-to-work law prohibiting workers from being forced to support a union as a condition of employment. The state’s new Republican governor, Eric Greitens, has vowed to sign it.

The Senate vote was 21-12. The measure would grandfather in existing union-management contracts that require workers to become union members or pay it a regular fee — provisions dubbed “security clauses” by unions. But all new union contracts will forbid this practice.