In the United States, the way in which a political party chooses a presidential candidate is up to the party itself. This is because there is no provision in the U.S. Constitution that calls for any particular method for choosing candidates. The two major parties in the U.S., the Republicans and the Democrats, each choose a presidential candidate at a national convention where delegates from each state cast votes. How the delegates vote usually is based on the results of primary elections or caucuses that were held in their respective states. The exact manner in which the delegates are chosen, the primaries or caucuses are held and how the delegates must vote is determined by each state's branch of that particular political party.
Within each party, every state is assigned a certain number of delegates. Other U.S. jurisdictions, such as the District of Columbia, Puerto Rico and American Samoa, also are assigned a specific number of delegates. These numbers are determined by the party and can be based on a combination of many factors, such as the state's population and its number of party members in the U.S. Congress. There also can be at-large delegates — usually current or former party officials — who are not obligated to vote for any particular candidate. In each state or jurisdiction, the party holds a convention where individuals are chosen to be delegates and attend the party's national convention to help choose a presidential candidate.
Primaries and caucuses are held in each state, usually starting in early January of the presidential election year. A primary is an election in which citizens cast secret ballots, and caucuses are meetings where votes are cast either publicly or by secret ballot. Local primaries and caucuses help determine the delegates to the state convention and which candidate or candidates those delegates will support. Just like at the state level, the exact manner in which this is done is up to the local branch of the party.
In some places, the percentage of delegates who are obligated to support a candidate is based on the percentage of votes received in the primary or caucus. Some primaries and caucuses, however, award all of the delegates to the candidate who received the most votes. At the national convention, the delegates cast their votes to choose a presidential candidate. The party's nominee might be the person who receives the most votes from delegates, or a majority of the votes might be required.
In some cases, the eventual nominee will already be known before the convention because he or she is assured of receiving more than enough votes from the party's delegates. When this happens, the other candidates from the party might endorse the front-runner and release their delegates to vote for him or her when the party begins the roll call to choose a presidential candidate. This typically is done as a sign of unity within the party, which is seen as giving the nominee a better chance in the general election than if the party were to have somewhat of a division within its ranks or doubt about its nominee.