In which type of primary can only a party's registered voters participate?

A closed primary allows only registered voters of a specific political party to participate in the primary election to select that party's candidates. This means that if you are not a registered member of the party holding the primary, you cannot vote in that election. The purpose of a closed primary is to ensure that the party’s candidates are chosen only by those who are dedicated to the party’s platform and ideology, thereby maintaining party unity and reducing the influence of outside or independent voters on the party's candidates.

In contrast, an open primary permits any registered voter to participate regardless of their party affiliation, enabling a broader voter base to influence candidate selection. A blanket primary lets voters choose candidates from any party for each office, and a run-off primary occurs if no candidate receives the required majority of votes in the initial primary, ultimately requiring a second election between the top candidates. Therefore, the defining characteristic of a closed primary is its exclusivity to the party's registered voters.