Depends on the state or the job. Right to Work" states, (right to work for less) generally don't have union Security clauses in their contracts. That means, if you want to be a freeloader, and get the benefits that union members get, without paying for them, you can. Some states require you to pay a like amount into a separate fund, (called agency fees) :
"Employees whose terms and conditions of employment are determined by a collective bargaining agreement may be required to pay an Agency Fee to their exclusive representative union. Under CA state law, Sections 3583-3585 of the Government code relating to higher education labor relations, eligible employees who do not pay union dues may be required to pay for the costs of "?lobbying activities designed to foster collective bargaining negotiations and contract administration or to secure for the represented employees advantages in wages, hours, and other conditions of employment in addition to those secured through meeting and conferring with the higher education employer." The decision of whether an Agency Fee will be levied rests solely with each individual union."
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