Missouri Constitution
Article III - Legislative Department
Section 12 Members of general assembly disqualified from holding other offices.

No person holding any lucrative office or employment under the United States, this state or any municipality thereof shall hold the office of senator or representative. When any senator or representative accepts any office or employment under the United States, this state or any municipality thereof, his office shall thereby be vacated and he shall thereafter perform no duty and receive no salary as senator or representative. During the term for which he was elected no senator or representative shall accept any appointive office or employment under this state which is created or the emoluments of which are increased during such term. This section shall not apply to members of the organized militia, of the reserve corps and of school boards, and notaries public.

Source: Const. of 1875, Art. III, Sec. 12.