Mo. Rev. Stat. § 115.628: Voter registration records — notification of registered voters of party affiliation opportunities — declaration of party affiliation.

1. The secretary of state shall maintain voter registration records in accordance with the Missouri voter registration system defined under section 115.158.

2. Local election authorities shall notify registered voters of the political party affiliation opportunities of this section using all current election mailings that would otherwise be mailed to registered voters prior to January 1, 2025.

3. Beginning January 1, 2023, the voter registration application form shall be amended to include a choice of political party affiliation.

4. Notwithstanding any other provision of law to the contrary, beginning January 1, 2023, voters may declare political party affiliation during the voter check-in process at any election. Appropriate software shall be provided at voter check-in for political party affiliation so as to minimize later data entry for election authorities. If the election authority does not use electronic poll books, then a signed, written notice in substantially the same manner as a change of address application is filed under section 115.165 is adequate. The election authority shall process this initial political party registration through its normal means of administration.

5. Notwithstanding any other provision of law to the contrary, all current processes for registering voters in the various counties shall remain in place.