Mo. Rev. Stat. § 128.340: New counties — assignment to congressional districts.

When any new county shall be organized, such county shall constitute and form a portion of the same congressional district embracing at the time the county or counties from which such new county shall be organized; and if any such county shall be organized from two or more counties lying in different congressional districts, then such county shall form and constitute a part of the congressional district embracing, at the time, the greater portion of the territory constituting such county thus organized.