Mo. Rev. Stat. § 115.021: Jurisdiction of election boards.

1. In each city which has over three hundred thousand inhabitants on January 1, 1978, and is situated in more than one county, the board of election commissioners for the city shall have jurisdiction in that part of the city situated in the county containing the major portion of the city.

2. In each county of the first class containing the major portion of a city which has over three hundred thousand inhabitants, the board of election commissioners shall have jurisdiction in that part of the county outside the city.

3. In each city not situated in a county, the board of election commissioners shall have jurisdiction throughout the city.

4. In all other counties, the election authority shall have jurisdiction throughout the county.