Mo. Rev. Stat. § 115.397: Voter may receive only one party ballot — voters not wishing a party ballot may vote for independents and on all propositions and questions.

In each primary election, each voter shall be entitled to receive the ballot of one and only one political party, designated by the voter before receiving his ballot. Each voter who participates in a party primary shall be entitled to vote on all questions and for any nonpartisan candidates submitted by political subdivisions and special districts at the primary election. Each voter who does not wish to participate in a party primary may vote on all questions and for any nonpartisan candidates submitted by a political subdivision or special district at the primary election.