Missouri Constitution
Article VIII - Suffrage and Elections
Section 2 Qualifications of voters—disqualifications.

All citizens of the United States, including occupants of soldiers’ and sailors’ homes, over the age of eighteen who are residents of this state and of the political subdivision in which they offer to vote are entitled to vote at all elections by the people, if the election is one for which registration is required if they are registered within the time prescribed by law, or if the election is one for which registration is not required, if they have been residents of the political subdivision in which they offer to vote for thirty days next preceding the election for which they offer to vote: Provided however, no person who has a guardian of his or her estate or person by reason of mental incapacity, appointed by a court of competent jurisdiction and no person who is involuntarily confined in a mental institution pursuant to an adjudication of a court of competent jurisdiction shall be entitled to vote, and persons convicted of felony, or crime connected with the exercise of the right of suffrage may be excluded by law from voting.

Source: Const. of 1875, Art. VIII, Sec. 2 (Amended November 4, 1958). (Amended November 5, 1974)