who determines the way in which primaries are set up
The presidential primary elections and caucuses held in the various states, the District of Columbia, and territories of the United States form part of the nominating process of candidates for United States presidential elections. The United States Constitution has never specified the process; political parties have developed their own procedures over time. Some states hold only primary elections, some hold only caucuses, and others use a combination of both. These primaries and caucuses are staggered, generally beginning sometime in January or February, and ending about mid-June before the general election in November. State and local governments run the primary elections, while caucuses are private events that are directly run by the political parties themselves. A state's primary election or caucus is usually an indirect election: instead of voters directly selecting a particular person running for President, they determine the number of delegates each party's national convention will receive from their respective state. These delegates then in turn select their party's presidential nominee. The first state in the United States to hold its presidential primary was New Hampshire in 1920.