In the event that there is a vacancy in both the offices of governor and lieutenant governor within the meaning of the constitution and the speaker of the house of representatives is unavailable, the president pro tempore of the senate, the secretary of state and the treasurer, shall, in the order named, act as governor until such vacancy is terminated, or until a governor is elected, or until such speaker or a preceding interim successor becomes available. (1959, No. 13, § 4, eff. March 4, 1959.)