http://en.wikipedia.org/wiki/Taft%E2%80%93Hartley_Act
Authorized individual states to outlaw union security clauses (such as the union shop) entirely in their jurisdictions by passing right-to-work laws. A right-to-work law prevents unions from negotiating contracts or legally binding documents requiring companies to fire workers who refuse to join the union. The amendments expressly excluded supervisors from coverage under the act, and allowed employers to terminate supervisors engaging in union activities or those not supporting the employer's stance.[8] The amendments maintained coverage under the act for professional employees, but provided for special procedures before they may be included in the same bargaining unit as non-professional employees.