Franchise in a primary or caucus is governed by rules established by the state party, although the states may impose other regulations.

While most states hold primary elections, a handful of states hold caucuses. Instead of going to a polling place, voters attend local private events run by the political parties, and cast their selections there. The advantage of caucuses is that the state party runs the process directly instead of having the state and local governments run them. The disadvantage is that most election laws do not normally apply to caucuses.

A handful of states practice non-binding "beauty contests", which are public opinion surveys for use by caucus delegates to select candidates to a state convention, which then in turn selects delegates to the national convention.

What is the difference between a binding and a non-binding primary?

In a binding primary, elected delegates are bound to vote for the candidate they pledged their support for, whereas in a non-binding primary, voters' wishes are considered advisory. Missouri's Presidential Preference Primary is a non-binding primary.