Final answer:
The difference between open and closed primaries lies in voter eligibility. Open primaries allow any registered voter to choose which party's primary to participate in, while closed primaries only allow registered party members to vote in their party's primary.
Step-by-step explanation:
The main difference between "open" and "closed" primaries is who is allowed to participate in the primary elections of a political party. In an open primary, any registered voter can select which party's primary they wish to participate in on the day of the election, regardless of their party affiliation, although they can only vote in one party's primary. In contrast, a closed primary is more restrictive; only those registered as party members can vote in their respective party's primary. This process aims to prevent individuals who may not support the party from influencing the selection of its nominee.
Some states also have variations such as semi-closed primaries, where unaffiliated voters can choose which primary to participate in, but registered party members can only vote in their own party's primary. In addition, states might employ semi-open primaries, where voters do not have to publicly declare their choice of party primary, or run-off primaries, which are used when no candidate wins a majority in the initial primary.