AP Syllabus focus:
‘Primaries select party nominees; open and closed primaries differ in who is eligible to vote in each party’s contest.’
Primaries are the main gateway to the general election ballot. Whether a state uses open or closed primaries shapes who participates, how candidates campaign, and how much control parties retain over nominations.
Core idea: eligibility rules determine who can choose a nominee
Primary election: An election in which voters choose a political party’s nominee for the general election.
Because states administer elections but parties have their own membership and goals, primary rules sit at the intersection of state election law and party autonomy.
This U.S. Election Assistance Commission visual categorizes partisan primary systems and briefly describes how voter eligibility differs across open, partially open/partially closed, and closed formats. It helps connect the abstract idea of “eligibility rules” to the concrete institutional types states use in practice. Source
Open primaries
In an open primary, voters are not required to be registered members of a party to participate in that party’s primary.
Open primary: A primary in which eligible voters may vote in a party’s nomination contest without formally registering with that party (rules vary by state).
Key features often include:
Broader access for independents and sometimes members of the other party
A choice on or near election day about which party’s primary to vote in (depending on state rules)
Weaker formal link between party membership and nominee selection
Closed primaries
In a closed primary, voters must be registered with a party in advance to vote in that party’s primary.
Closed primary: A primary in which only voters registered with a political party may vote in that party’s contest.
Common characteristics include:
Stronger party control over who helps pick the nominee
A clearer connection between party identifiers and nomination outcomes
Registration deadlines that can exclude late-deciding voters
Key effects and trade-offs
Participation and access
Open primaries generally lower participation barriers by expanding eligibility, especially for independent voters. Closed primaries can reduce participation among voters who dislike party labels or miss registration deadlines, even if they are politically engaged.
Candidate incentives and campaign strategy
Eligibility rules can influence which voters candidates prioritise.
In more open systems, candidates may be pulled toward broader appeal, messaging to independents and occasional voters.
In closed systems, candidates may focus on core partisans, who are more likely to be ideologically consistent and politically active.
Strategic voting (crossover and raiding concerns)
Open primaries can create incentives for strategic voting, such as:
Voting in the other party’s primary to influence the opposing nominee
Attempts at “raiding,” where groups try to nominate a weaker opponent
Closed primaries reduce, but do not eliminate, strategic behaviour because eligibility is tied to party registration.
Party rights vs. state rules
Because primaries help determine party nominees, parties may argue they have a constitutional freedom of association interest in deciding who participates. States, meanwhile, justify rules based on goals like higher turnout, administrative simplicity, or broader voter inclusion. Court disputes in this area often turn on whether a state’s primary system significantly interferes with a party’s ability to define its membership and select its standard-bearer.
Practical administration
States must decide how voters access a ballot:
Separate party ballots vs. a single combined ballot
Whether a voter’s party choice becomes a public record
How absentee or early voting interacts with party selection rules
Variations you may encounter
Not all states fit neatly into “open” or “closed.”
Semi-closed: independents may choose a party primary, but registered partisans cannot cross over.
Semi-open: voters may choose a party primary, but their choice may update or disclose party affiliation under state rules.
FAQ
Not necessarily.
In some states, voters choose a party ballot each primary election without a lasting affiliation. In others, the act of choosing a party ballot may be recorded and can affect future eligibility rules.
States use different approaches.
Some fully exclude independents from party primaries.
Others use a semi-closed model allowing independents to opt in to a party’s primary without permanent party registration.
Crossover voting is when a voter participates in the other party’s primary.
It can be strategic (influencing the opponent’s nominee) or sincere (supporting a preferred candidate across party lines), depending on voter motives and the competitiveness of each party’s race.
Sometimes, but it depends on state law and party rules.
Parties may challenge primary systems that they argue significantly burden their associational rights, or they may negotiate rule changes through state legislation and party-state agreements.
Election administrators typically require voters to select a party ballot before the ballot is issued.
That choice may be made on an application, through an online portal, or at an elections office, and deadlines can affect access differently in open versus closed systems.
Practice Questions
(2 marks) Describe one key difference between an open primary and a closed primary.
1 mark: Identifies that open primaries allow non-party members/independents to vote in a party contest.
1 mark: Identifies that closed primaries restrict voting to registered party members (usually registered in advance).
(6 marks) Explain two ways that open versus closed primaries can affect (a) political participation and (b) party control over nominations.
Up to 3 marks (Participation): Explains how eligibility rules can increase/decrease participation (e.g., independents included in open primaries; registration barriers in closed primaries) with clear linkage to turnout/access.
Up to 3 marks (Party control): Explains how open primaries can dilute party members’ influence and enable crossover voting, while closed primaries strengthen party members’ gatekeeping over nominees.
