Respond to the following hypothetical with a thorough analysis of the relevant facts as they apply to the law.

Martin and Harris are employed by Stith Mfg. Co., but are not members of the union that represents the Stith production workers. However, they are required to pay an agency fee to the union under the collective agreement between the union and the employer. A portion of the agency fees they pay go into a "strike pay" fund that is used to support workers who go on strike.

When negotiations to renew the expiring collective agreement break down, the union votes to go on strike. Harris and Martin oppose the strike, and continue working during the strike. They also refuse to pay the portion of the agency fee that goes to the strike pay fund. Can the union require that they pay the portion of the agency fee that is paid into the strike pay fund?