Primary systems explained
The answer
Not all primaries work the same way, and the difference decides who gets a say in picking each party's nominee. In a closed primary, only registered party members can vote in that party's contest. In an open primary, any registered voter can pick either party's ballot, no party registration required. Most states fall somewhere between those extremes, and a handful, California, Washington, and Alaska, don't sort candidates by party at all before the general election.
How it actually works
NCSL groups state primary rules into six categories: closed (party members only), partially closed (parties can choose to let unaffiliated voters in each cycle), partially open (any voter can cross party lines but must publicly declare a ballot choice, as in Illinois, Ohio, and Iowa), open to unaffiliated voters only (New Hampshire, Colorado), fully open (any voter privately picks any party's ballot), and top-two or top-four systems that skip party primaries entirely.1
California adopted its top-two system through Proposition 14 in 2010: every candidate from every party appears on one primary ballot, and the top two vote-getters advance to November, even if they're from the same party. Alaska went further in 2020 with Ballot Measure 2, creating a top-four primary that feeds into a ranked-choice general election (see ranked-choice-voting-explained).2 Louisiana used to run the reverse of a primary altogether, a jungle primary where everyone competed on Election Day itself with a December runoff if no one won a majority. New state laws end that system starting with the May 2026 primary for Congress, the state Supreme Court, and several other offices, replacing it with closed party primaries.3
Who holds the power
State legislatures write these rules, not the parties, though parties can sometimes opt into a more open or closed format where state law allows it, as with partially closed primaries. Changing a state's primary system usually requires either a legislative vote or a citizen ballot initiative (see how-ballot-initiatives-work).
Where you come in
Check your state's specific primary type before you register. Some states require you to declare a party affiliation weeks or months before the primary to be eligible to vote in it. If you're registered as unaffiliated or independent, find out whether your state lets you vote in a party primary at all. See how-to-read-your-ballot for how your ballot reflects your state's system.
Reform is active right now. New Mexico's SB 16, signed in 2025, opens major-party primaries to roughly 330,000 unaffiliated voters starting in 2026. Alaska faces a signature-backed repeal campaign targeting its top-four system for the 2026 ballot. Oklahoma and Michigan both have citizen initiatives in circulation for 2026 that would change their primary or nomination systems.3
Related
Glossary: closed-primary, top-two-primary, jungle-primary, crossover-voting
See also: why-the-primary-is-the-real-election