820 ILCS 12/5
It is the policy of the State of Illinois that employers, employees, and their labor organizations are free to negotiate collectively. It is also the policy of the State of Illinois that employers, employees, and their labor organizations may freely negotiate union security agreements, including, but not limited to, those requiring dues to be paid to a labor organization as permitted under 29 U.S.C. 158 (a)(3). It is further the policy of the State of Illinois that no local government or political subdivision may create or enforce any local law, ordinance, regulation, rule, or the like that by design or application prohibits, restricts, tends to restrict, or regulates the use of union security agreements between a labor organization and an employer as permitted under 29 U.S.C. 158 (a)(3).
820 ILCS 12/5