Open vs. Closed Primaries

In a closed primary, only voters who have registered with a party may participate in that party's primary. A registered Republican in New York cannot vote in the Democratic primary, and vice versa. Independents and unaffiliated voters are excluded entirely from both parties' primaries.

Proponents of closed primaries argue that parties are private organizations with the right to choose their own nominees without outside interference. Allowing non-members to vote in a primary could enable strategic voting — members of one party voting in the other's primary to nominate the weakest possible opponent in the general election. While the scale of such cross-party raiding is disputed in the research, it remains a theoretical concern.

The main criticism of closed primaries is that they exclude the roughly 30-49% of voters who are registered as independent or unaffiliated — leaving them with no say in who the major parties nominate. In heavily partisan states (deep blue or deep red), the primary is often the only competitive election, so closing it to independents effectively disenfranchises a large share of the electorate from the only vote that matters.

In an open primary, any registered voter may participate in either party's primary, regardless of their own party registration. In states like Wisconsin and Michigan — key swing states — voters simply ask at the polling place for the party ballot they want to vote on.

Research on the moderating effect of open primaries is mixed. Theoretically, by allowing independents and cross-party voters to participate, open primaries should produce more centrist nominees who can appeal to a broader coalition. In practice, the effect appears modest: primary electorates in open primary states are still dominated by the party's base voters, who are more ideologically engaged and more likely to turn out in low-information primary elections.

Open primaries matter most in presidential nomination contests where an outsider candidate is running. Bernie Sanders benefited significantly from open primaries in 2016 and 2020, drawing support from independents who were enthusiastic about his campaign but might not have been registered Democrats. Trump similarly benefited from cross-party interest in his 2016 campaign, particularly in open primary states.

California adopted the top-two primary system via Proposition 14 in June 2010, going into effect in 2012 for state and federal offices (excluding presidential primaries, which use separate party elections). Under the top-two system, all candidates from all parties appear on a single June primary ballot. Every registered voter receives the same ballot. The two candidates with the most votes advance to the November general election — regardless of their party affiliation.

This creates the possibility of same-party general elections. In 2016, California's US Senate race featured two Democrats — Kamala Harris and Loretta Sanchez — in the November general election after both outpolled the Republican candidates in the June primary. This happened again in 2024 when Democrats Adam Schiff and Steve Garvey (a Republican) advanced; Garvey had benefited from Republican voters concentrating behind a single candidate while Democratic votes split among multiple candidates.

The Louisiana top-two system has an additional feature: if a candidate wins an outright majority (50%+1) in the primary, they win without a general election. This is how some deeply one-party Louisiana races are decided in October rather than November. Washington state uses a system similar to California's without the Louisiana outright-majority escape valve.