Political parties are essential to the functioning of democracy in the United States. They serve to reflect the views of their constituents and organize various political agendas. One significant aspect of political parties is the role they play in elections, specifically through the process of primary elections. Primary elections serve as a means for parties to select their candidates for general elections. These primaries can take several forms, including open, closed, and semi-closed primaries. Given this context, what is the primary function of a closed primary election?