State laws governing primaries can be confusing. In some states, political parties control their primaries, including whether to hold a primary at all. Elsewhere, primaries are mandatory. Lawmakers and voters also frequently amend primary rules, whether making minor changes or significant ones. 

To understand the procedures governing primary elections, a voter needs to understand what state law says, what party rules say, and what this all means in practice.

Our new primary type hub page includes improved explanations and classifications of state primary laws and voter participation rules. This page covers two main topics: the laws that govern primary participation and outcomes, and the rules that major parties in each state use for their primaries.

Primaries background

Most American voters are familiar with primaries, which allow voters to determine which candidates compete in the general election. The first primary in the U.S. occurred in Wisconsin in 1903. By the end of World War I, all but four of the then 48 states had some form of legally-regulated and taxpayer-funded nominating contests. 

The system of primaries across the United States is uniquely American. Most other democracies do not have legally-regulated and taxpayer-funded primaries. Additionally, state laws governing primaries vary significantly, differing in who may participate in party primaries, how much say state parties have in running primaries, and the threshold to advance to the general election. Primaries can also vary by office, with local, judicial, and presidential primaries often having entirely different rules from other state primaries.

Primaries in the United States

Forty-four states require primaries for all statewide offices. Alabama, South Carolina, and Virginia allow parties to select their nominees without a primary. 

Looking at laws governing who may participate in a primary, we classify states into one of five categories: 

• Open: State law says that anyone can vote in this primary, regardless of partisan affiliation. There are no affiliation requirements before or after the primary.
• Closed: State law says a voter must be affiliated with a political party to participate in its primary. Note: In some states with closed primaries, it may still be possible for a voter to affiliate or change affiliation on the day of the primary.
• Semi-closed: State law says that voters who are not affiliated with a political party may participate in the partisan primary of their choice while retaining their status as an unaffiliated voter.
• Top-two or variation: State law says that all registered voters vote in the same primary, and all candidates are listed on the same ballot.
• Party discretion: State law permits parties to choose participation rules for their primaries.

Thirty-nine states require major parties to hold an open, closed, semi-closed, or top-two style primary for most congressional and state offices. Eleven states permit parties to choose between at least two possible primary types for most primaries. Of the 39 states where state law specifies a single primary type: 

1. Fourteen require open primaries.
2. Thirteen require closed primaries. 
3. Nine require semi-closed primaries.
4. Three require top-two style primaries. 

Changing laws

This year, New Mexico Gov. Michelle Lujan Grisham (D) signed SB 16 into law, moving the state from a closed primary system to semi-closed. Maine also moved from a closed to a semi-closed primary though LD 213 in 2021, and Colorado did so in 2016 when voters approved Proposition 108 53.3%-46.7%. Alaska changed its primary type in 2020 when voters approved Measure 2 50.5%- 49.5%, moving to a top-four primary. In 2023, North Carolina, lawmakers removed the ability of political parties to decide whether unaffiliated voters may participate in party primaries, requiring them to be able to participate through S 747.

Primaries in use

So what types of primaries do states actually use? In 40 states, both the Democratic and Republican parties use the same primary system for most or all statewide offices. In seven states, the parties operate different types of primaries. In Alaska, California, and Washington, there are no party primaries for congressional and state-level offices, and all partisan candidates compete in the same primary.  

The Democratic Party uses:

1. Open primaries in 18 states
2. Closed primaries in 16 states
3. Semi-closed primaries in 14 states

The Republican Party uses:

1. Open primaries in 15 states
2. Closed primaries in 23 states
3. Semi-closed primaries in nine states

Several examples illustrate the need to keep state law, party rules, and other details in mind in order to understand a state’s primary system.

While Tennessee requires voters to be affiliated with a party to participate in its primary, the state doesn’t have partisan voter registration. A voter must either be “a bona fide member” of the party or “[declare] allegiance to the political party” at the time of the primary.

In West Virginia, the Republican Party will hold a closed primary in 2026 — a change from previous elections in which it allowed unaffiliated voters to participate — while the West Virginia Democratic Party will continue letting independents vote in its nominating contests.

In Maryland, state law would permit non-party members to vote in party-run primaries, but neither the Democratic nor the Republican parties allow them to do so. 

For more information on the complex landscape of primaries, check out Ballotpedia’s hub for statewide primaries, and check out our Election Administration Legislation Tracker to stay up to date on potential primary changes.