For as long as Missouri has had voter registration, there has been no requirement that people choose a political party as part of the process.
That means every primary is open to every voter. Every four years, it is possible to cast a ballot in one party's presidential primary in the spring and another party's primary for governor in August.
And it also means anyone who wants to be a candidate for a partisan public office doesn't have to do anything more than pay a fee to the party of their choice on filing day.
Under legislation proposed for the session that begins Wednesday, that would change. Two House members, Reps. Dan Stacy, R-Blue Springs, and Jered Taylor, R-Republic, along with Sen. Andrew Koenig, R-Manchester, have introduced bills requiring voters to choose a party if they wish to vote in primaries.
And anyone who wants to run for office in a particular party would have to be registered in that party for at least 23 weeks prior to the opening of filing for office.
"I just think it makes sense that members of the same party should select their candidates," Koenig said in an interview.
The interest in primaries waxes and wanes with the number of competitors for a high-profile office and whether there is a controversial ballot issue before voters.
The highest turnout primary, in terms of total votes, was the 2016 presidential primary. With hot contests in both major parties, 1.57 million people voted and almost 60 percent were Republicans.
The highest turnout primary for governor in recent years was in 2004, when 1.45 million voters cast ballots and 58 percent chose the Democratic ballot for the contest between State Auditor Claire McCaskill and incumbent Gov. Bob Holden.
Right-to-work and Medicaid expansion were the big draws for primary voters in 2018 and last year. More than 1.2 million votes were cast in August of each year with no significant contests for top offices.
While those primaries had more total votes, the share of registered voters who participated in primaries was much higher in the 1960s and 1970s than it is today. While up to one-third of registered voters cast ballots at that time, in recent years as few as 20 percent have turned out.
The proposals to limit nominating primaries to voters registered in a party would still allow independents to receive a non-partisan ballot for statutory and constitutional questions.
Changing the current system means bucking 155 years of tradition in state politics. That's a tough job, Koenig admitted.
"I am not under the belief that everybody wants this," Koenig said.
Missouri shares an open primary system with 14 other states, according to a tracking page maintained by the National Conference of State Legislatures. There are nine states that have closed primaries where only people registered in political parties may vote.
Some states allow parties to choose year-to-year whether to allow independent voters to participate in their primaries and some states have systems that always allow independent voters to select a primary.
A few have completely eliminated partisan primaries. California and Washington have primaries that send the top two vote-getters to the November election, and Alaska just implemented a top-four system.
If passed, the closed system would be fully implemented for the 2026 primaries. Voters who do not file a new registration card would be assigned to a political party based on their ballot choices in primaries in 2022 and 2024.
The changes are likely to be resisted, said Peverill Squire, professor of political science at the University of Missouri.
"My guess is that it probably won't be particularly popular because people don't like changing the procedures with which they are familiar," Squire said.
One of the fears associated with a push to close primaries is that voters who do not adhere to a party's views will cast ballots in a primary to defeat a strong general election contender. In the 2014 primary for St. Louis County Executive, Republican candidates Rick Stream and Tony Popousa urged Republicans to vote in their primary.
Stream told St. Louis Public Radio that some Republicans said they wanted Democrats to nominate the weakest candidate for the fall election.
Changing a primary outcome by enlisting people to vote in a primary for a party they did not intend to support in the fall campaign would be difficult, Squire said.
"It is really hard to coordinate that sort of thing and people's behavior in a sophisticated plot," he said. "People generally know which party they are supportive of and vote in primaries because they are interested in the decisions of that party."
Closing the primaries, Squire said, would likely have little impact on the results. And it might upset some voters.
"You do have a lot of people who are indifferent to political parties and like to think of themselves as independent," he said.
In the interview, Koenig noted it was the third year he has filed the bill. The first year nothing happened, he said, and last year it received a committee hearing.
He's hoping to get it much further this year.
"It is just like any organization," he said. "You don't want people outside that organization picking your leaders."
The Missouri Independent is a nonprofit, nonpartisan news organization covering state government and its impact on Missourians.