Missouri Revised Statutes Title XVIII. Labor and Industrial Relations § 290.590. Labor organization membership, dues, and fees not required as condition of employment--definitions--violations, penalty--investigation of complaints--inapplicability, when

(1) “Employer”, any individual, organization, partnership, state agency, political subdivision, corporation, or other legal entity which employs or has employed one or more individuals performing services for the entity within this state; and

(2) “Labor organization”, any organization of any kind or agency, or employee representation committee or union which exists for the purpose in whole or in part of dealing with employers concerning wages, rates of pay, hours of work, other conditions of employment, or other forms of compensation.

(1) Become, remain, or refrain from becoming a member of a labor organization;

(2) Pay any dues, fees, assessments, or other similar charges however denominated of any kind or amount to a labor organization; or

(3) In lieu of the payments listed under subdivision (2) of this subsection, pay to any charity or other third party any amount equivalent to, or on a pro rata basis, any dues, fees, assessments, or other charges required of members of a labor organization.

5. (1) Any person injured as a result of any violation or threatened violation of this section shall be entitled to injunctive relief against any and all violators or persons threatening violations.

(2) Any person injured as a result of any violation or threatened violation of this section may recover any and all damages of any character resulting from such violation or threatened violation including costs and reasonable attorney fees. Such remedies shall be independent of and in addition to the other penalties and remedies prescribed under this section.

(1) To employers and employees covered by the federal Railway Labor Act;

(2) To federal employers and employees;

(3) To employers and employees on exclusive federal enclaves;

(4) Where this section conflicts with or is preempted by federal law; or

(5) To any agreement between an employer and a labor organization entered into before August 28, 2017, but shall apply to any such agreement upon its renewal, extension, amendment, or modification in any respect after August 28, 2017.