Voter turnout is projected at nearly 1.1 million people, or 27 percent, Tuesday.
More than four million people are registered to vote in Missouri.
Tuesday's primary will feature state, federal and county races as well as five proposed constitutional amendments.
In Kansas, Secretary of State Kris Kobach estimates only 22 percent of the state's registered voters will take part in the upcoming primary election.
Kobach said voter turnout may be as much as 27 percent in the Fourth Congressional District of Kansas where former Congressman Todd Tihart is trying to take back his old seat from incumbent Congressman Mike Pompeo.