Each of the 50 U.S. states, the District of Columbia, and five territories of the United States hold either primary elections or caucuses to help nominate individual candidates for president of the United States. This process is designed to choose the candidates that will represent their political parties in the general election.

The United States Constitution has never specified this 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 a candidate will receive from their respective state for each party's national convention. These delegates then in turn select their party's presidential nominee. The first state in the United States to hold its presidential primary was North Dakota in 1912, following on Oregon's successful implementation of its system in 1910.

Each party determines how many delegates it allocates to each state. Along with those "pledged" delegates chosen during the primaries and caucuses, state delegations to both the Democratic and Republican conventions also include "unpledged" delegates who have a vote. For Republicans, they consist of the three top party officials who serve At Large from each state and territory. Democrats have a more expansive group of unpledged delegates called "superdelegates", who are party leaders and elected officials. If no single candidate has secured an absolute majority of delegates (including both pledged and unpledged), then a "brokered convention" occurs: all pledged delegates are "released" after the first round of voting and are able to switch their allegiance to a different candidate, and then additional rounds take place until there is a winner with an absolute majority.

The staggered nature of the presidential primary season allows candidates to concentrate their resources in each area of the country one at a time instead of campaigning in every state simultaneously. In some of the less populous states, this allows campaigning to take place on a much more personal scale. However, the overall results of the primary season may not be representative of the U.S. electorate as a whole: voters in Iowa, New Hampshire and other less populous states which traditionally hold their primaries and caucuses in late-January/February usually have a major impact on the races, while voters in California and other large states which traditionally hold their primaries in June generally end up having a significantly reduced say because the races are usually over by then. As a result, more states vie for earlier primaries, known as "front-loading", to claim a greater influence in the process. The national parties have used penalties and awarded bonus delegates in efforts to stagger the system over broadly a 90-day window. Where state legislatures set the primary or caucus date, sometimes the out-party in that state has endured penalties in the number of delegates it can send to the national convention.

There is no provision for the role of political parties in the United States Constitution, since the Founding Fathers did not originally intend for American politics to be partisan. In Federalist Papers No. 9 and No. 10, Alexander Hamilton and James Madison, respectively, wrote specifically about the dangers of domestic political factions. Thus in the first two presidential elections, the Electoral College handled the nominations and elections in 1789 and 1792 that selected George Washington. The beginnings of the American two-party system then emerged from Washington's immediate circle of advisors. Hamilton and Madison, who wrote the aforementioned Federalist Papers against political factions, ended up being the core leaders in this partisanship: Hamilton became the leader of Federalist Party while Madison co-helmed the Democratic-Republican Party with Thomas Jefferson.

Starting with the 1796 election, congressional party or a state legislature party caucus selected the party's presidential candidates. Before 1820, Democratic-Republican members of Congress would nominate a single candidate from their party. That system collapsed in 1824, and since 1832 the preferred mechanism for nomination has been a national convention.

The first national convention to nominate a presidential candidate was called by the Anti-Masonic Party in 1831, as they could not use the caucus system because they had no congressmen. The party leaders instead called for a national meeting of supporters to set the party's candidate. This convention was held in Baltimore, Maryland, on September 26, 1831, which selected William Wirt as their presidential candidate.

Delegates to the national convention were usually selected at state conventions whose own delegates were chosen by district conventions. Sometimes they were dominated by intrigue between political bosses who controlled delegates; the national convention was far from democratic or transparent.