In the United States, political parties nominate candidates to represent them in elections. Candidates seek party nominations to access the party's resources such as funding, volunteers, membership lists, ballot access, and perceptual legitimacy. Parties offer nominations to increase their influence over elected officeholders, thereby advancing their policy priorities.

There are several models by which parties nominate candidates in the United States. The two principal models are the open primary election and the closed primary election.

In the open primary election, any voter may cast a ballot to nominate a candidate as the candidate of a designated political party. The perceived advantage of this system is that it increases voter access to the political process. The perceived disadvantage of this system is that it loosens party control over who represents them, meaning nominated candidates may not actually reflect the will of the party's leadership and members.

In the closed primary system, only voters who are registered members of a political party may cast a ballot to nominate a candidate as the candidate of a designated political party. This increases party control over the nominating process and ensures that a party's candidates are in the greatest amount of ideological symmetry with the party itself. However, it decreases opportunities for political participation by the public at-large.