in the United States, an election to select a political party's candidates to run for public office. Primaries may be closed (partisan), allowing only declared party members to vote, or open (nonpartisan), enabling all voters to choose which party's primary they wish to vote in without declaring any party affiliation. Primaries may be direct or indirect. A direct primary, which is now used in some form in all U.S. states, functions as a preliminary election whereby voters decide their party's candidates. In an indirect primary, voters elect delegates who choose the party's candidates at a nominating convention.

Indirect primaries for the presidency of the United States (presidency of the United States of America) are used in many states. Voters in these elections generally select delegates who attend a national political convention and are bound and pledged to cast their ballots on the basis of the preferences of the voters. Delegates may be bound for only one convention ballot or until they are released by the candidate. In some states, the presidential preference vote is advisory and does not bind the delegates. Rules for selecting delegates are determined by the political parties and vary by state. Delegates can be selected on a winner-take-all basis—as in many Republican Party state primaries, in which the candidate who wins the most votes wins all the delegates at stake—or by proportional representation—as in the Democratic Party primaries, in which any candidate receiving a percentage of the votes above some threshold is entitled to at least one delegate. Allocating delegates by proportional representation makes it difficult for a candidate to build a delegate landslide out of a series of narrow primary victories, and Democratic presidential contests usually have taken longer to select a clear front-runner. In an attempt to enhance the power of Democratic party leaders and elected officials and to minimize the influence of the primaries, during the 1980s the Democratic Party created so-called “superdelegates,” a group of unelected and unpledged delegates that included members of the Democratic National Committee, Democratic governors, and Democratic members of the U.S. House of Representatives (Representatives, House of) and Senate.

The formal, legally regulated primary system is peculiar to the United States. The earliest method for nominating candidates was the caucus, which was adopted in colonial times for local offices and continued into the 19th century for state and national offices. Party conventions were instituted as a means of checking the abuses of the caucus system but also became subject to abuses, which led first to their regulation and ultimately to their elimination for most offices except president and vice president. After 1890, mandatory regulations transformed the primary into an election that is conducted by public officers at public expense.

Although direct primaries were used as early as the 1840s, the primary system came into general use only in the early 20th century. The movement spread so rapidly that by 1917 all but four states had adopted the direct primary for some or all statewide nominations. For the presidential contest, however, primaries fell into disfavour and were generally used in fewer than 20 states until the 1970s, after which most states adopted primaries. Attention from the news media has increased the importance of presidential primaries to the point where success—especially in New Hampshire (which usually has held the first presidential primary) and in other early primaries—gives a candidate a great advantage in publicity and private campaign funding, whereas failure can end a campaign.

The merits of open versus closed primaries have been widely debated. Proponents of open primaries argue that voters should be able to choose which primary they will vote in at each election. Open primaries allow participation by independents unwilling to declare a party affiliation to vote and prevent intimidation of voters who wish to keep their affiliation private. Party organizations prefer closed primaries because they promote party unity and keep those with no allegiance to the party from influencing its choice, as happens in crossover voting, when members of rival parties vote for the weakest candidate in the opposition's primary. Several states have adopted variations, including the mixed primary, which allows independents to vote in either party's primary but requires voters registered with a political party to vote in their own party's primary. Following legal challenges (particularly by the Democratic and Republican parties), some variations, such as Washington's blanket primary, which enabled voters to select one candidate per office irrespective of party affiliation, and California's so-called “jungle primary” (a variation of the blanket primary), in which voters chose one candidate per office but only the votes of registered party voters were counted in determining the winner of each party's primary contest, were declared unconstitutional in the early 21st century.

Although primaries are uniquely American, there are some parallels in other countries. For example, the Australian Labor Party has used a “preselection” ballot, in which candidates in each locality have been selected by party members in that locality from those offering themselves for the preselection vote.