Before the mid-20th century, political parties seldom used primary elections to nominate state candidates for office; instead, they relied on other processes, such as caucuses or conventions. Political pressures arising in the 1950s led many states and parties to begin using primaries to select their nominees. Although almost all states now use a primary system for legislative elections, the way they hold their primaries differs nationwide. (The adoption of primaries to select presidential nominees followed in the 1970s, and is not the topic of this webpage.)
The table below shows the different types of primary elections states have used since 2000, broken into NCSL’s five main categories:
Closed: Only registered members of a political party may participate in the primary.
Semi-Closed: Parties may allow unaffiliated voters to participate in their primaries, and may change their decision from one election cycle to the next.
Open to Unaffiliated Voters: Unaffiliated voters may participate in one party’s primary.
Partially Open: Voters may participate in any party’s primary, but participation in a primary constitutes registration as a member of that party.
Open: Any voter may participate in any party’s primary. In these states, voters are not registered as members of a particular party, though they may be asked to attest that they are a “bona fide” member of the party in whose primary they choose to participate.
For more information on each primary type, including special cases like Louisiana (All-Comers), Alaska (Top Four) or California and Washington (Top Two), visit our State Primary Election Types webpage.
Lastly, some states previously used a blanket primary, in which all party nominations for all offices were contested on a single ballot. Voters in a blanket primary could, for example, choose the Democratic nominee for president, the Republican nominee for governor, the Libertarian nominee for attorney general and so on. That system was declared unconstitutional by the Supreme Court in Democratic Party of California v. Jones and has not been used since the early 2000s.
This page is part of NCSL’s Toolkit for Primary Elections, which provides information on many different aspects of party nomination systems.