Wash. Rev. Code § 49.66.030
An employee association shall be deemed the properly designated representative of a bargaining unit when it can show evidence that bargaining rights have been assigned to it by a majority of the employees in the bargaining unit. Should questions arise concerning the representative status of any employee organization claiming to represent a bargaining unit of employees, upon petition by such an organization, it shall be the duty of the director, acting by himself or herself or through a designee to investigate and determine the composition of the organization. Any organization found authorized by not less than thirty percent of the employees of a bargaining unit shall be eligible to apply for an election to determine its rights to represent the unit. If more than one organization shall claim to represent any unit, the director, or his or her designee, may conduct an election by secret ballot to determine which organization shall be authorized to represent the unit. In order to be certified as a bargaining representative, an employee organization must receive, in a secret ballot election, votes from a majority of the employees who vote in the election, except that nothing in this section shall prohibit the voluntary recognition of a labor organization as a bargaining representative by an employer upon a showing of reasonable proof of majority. In any election held pursuant to this section, there shall be a choice on the ballot for employees to designate that they do not wish to be represented by any bargaining representative. No representation election shall be directed in any bargaining unit or any subdivision thereof within which, in the preceding twelve-month period, a valid election has been held. Thirty percent of the employees of an employer may file a petition for a secret ballot election to ascertain whether the employee organization which has been certified or is currently recognized by their employer as their bargaining representative is no longer their bargaining representative.
No employee organization shall be certified as the representative of employees in a bargaining unit of guards, if such organization admits to membership, or is affiliated directly or indirectly with an organization which admits to membership, employees other than guards. The determination shall be based upon a plurality of votes cast in such election, and shall remain in effect for a period of not less than one year. In determining appropriate bargaining units, the director shall limit such units to groups consisting of registered nurses, licensed practical nurses or service personnel: PROVIDED, HOWEVER, That if a majority of each such classification desires inclusion within a single bargaining unit, they may combine into a single unit.
RCW 49.66.030