Open vs. Closed Primaries: How Election Rules Shape Who Votes

With fewer and fewer competitive general elections, primaries are now more consequential than ever in determining the makeup of Congress and who represents us. Yet few people show up to these contests. In recent midterm primary cycles, only 1 in 5 eligible voters usually participate. This raises obvious concerns that the 20% of voters making decisions on behalf of all Americans do not look like or hold the same views as all those eligible to participate. 

Perhaps the most consequential policy lever to address low participation and unrepresentative primary electorates are the rules for who is allowed to participate in these elections. Some states only allow those previously registered with a party to participate in that party’s primary elections. In these closed contests, voters unaffiliated with a party are not allowed to participate. 

If voters wish to vote for a different party than they are currently registered for, they sometimes have to make this switch months before the election takes place — before they know who will be on the ballot. Other states require voters who have declared a party affiliation to vote that party’s ballot, but allow unaffiliated voters to pick which party ballot they wish to vote. So-called “open” states allow all voters to pick which party’s nominating contest to participate in at the polls, and a few states run nonpartisan primaries, where all candidates regardless of party affiliation appear on the primary ballot. Finally, some states delegate these rules to the formal party organizations, leading to differing rules for participation for each party and even each office contested. 

States and parties have continuously tinkered with their rules for participation in nominating contests, but they have generally shifted from holding closed primaries to allowing unaffiliated voters to participate. 

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In 2000, 36% of all primary elections were closed to unaffiliated voters. By 2024, this has dropped to 31% of all primary elections. Primaries open to unaffiliated voters have increased from 17% in 2000 to a quarter of all primary contests in 2024. The percentage of contests that are open to all registered voters has gone down slightly from 32% to 29% over the past quarter-century. Finally, more states are experimenting with nonpartisan primary rules. In 2000, three states used nonpartisan primaries for at least some of their elections: California, Nebraska, and Washington. Alaska joined this group in 2022, and many other states are considering such changes. 

State Democratic parties have generally been more likely to open their contests to unaffiliated voters. State Republican parties were also moving in this direction. Now, several states are reversing this trend. Wyoming no longer allows voters to change party affiliation on Election Day, and Ohio and Texas are attempting to implement closed primary systems.

Just how unrepresentative are primary voters? And do the rules for primary participation affect who shows up? 

Previous studies have attempted to answer these questions with inconclusive results. They have mostly relied on data for only a handful of election years and surveys of a small fraction of voters to draw their conclusions. I can now give more definitive answers for three reasons. First, I use a more accurate record of the rules for primary participation for every contest since 2000, taking into account differences by party and office within each state. Second, I use each state’s voter file to identify the makeup of the electorate, which records who voted in each election. Third, panel data and modern techniques for causal inference allow me to be confident that observed changes in a state’s turnout and electorate composition after a rule change is attributed to the rule change itself, rather than other factors. 

First, I find that primary voters are not at all representative of the pool of general election voters, and even less so of the pool of registered voters. The average age of primary voters is 61 years old, compared with 55 years old for general election voters and 48 years old for eligible voters. The typical state’s primary electorate is 15% non-white, compared with 18% for general electorates and 26% for eligible electorates. Unaffiliated voters are severely underrepresented in primary elections. They make up only 10% of the average state’s primary contest, compared with 22% in general elections and 34% among all eligible voters. Unaffiliated voters tend to be more ideologically moderate, on average, than party-affiliated voters. This means that primary contests likely include far more voters at the ideological extremes and far fewer centrist voters than general contests — the precise moment when the eventual officeholder is usually selected.

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What happens when states change their rules for primary participation? 

When states open their primaries to unaffiliated voters, voter turnout increases by 5 percentage points on average. Considering the typical primary only sees 20 percent of voters participating, this equates to handing out 25 percent more “I voted” stickers. The gains are even larger when switching from closed to open (6 percentage point increase) or from closed to nonpartisan (almost 11 percentage point boost). 

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Unsurprisingly, allowing unaffiliated voters to participate sharply increases their share of the electorate. The average state switching from a closed rule to an open-to-unaffiliated rule sees a 14 percentage point increase in the percentage of the electorate that is unaffiliated. This means that when unaffiliated voters are invited to participate, many of them accept that offer. Opening primaries to unaffiliated voters also increases the share of the electorate that is nonwhite, and in particular Latino voters. This makes sense given that a higher percentage of Latinos do not affiliate with a party, compared to other racial or ethnic groups. Finally, switching from a closed primary rule to open to unaffiliated, open, or nonpartisan decreases the average age of the primary electorate by one to two years. 

These shifts in who participates makes the primary electorate much more representative of the pool of general election voters in terms of race or ethnicity, party affiliation, and age. Given the strong relationship between party affiliation and ideological leanings, it is also likely that a more moderate electorate shows up at the polls once they are allowed to do so. 

What does this mean going forward? 

States that decide to open their primaries to unaffiliated voters or switch to open or nonpartisan primaries are likely to see increased participation and primary voters who look and vote more like general election voters than they currently do. The opposite is true of states that decide to exclude unaffiliated voters from nominating contests. Turnout will decline, and the electorate will look whiter, older, more partisan, and more extreme as a result.

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