The Constitution says nothing about political parties or how they choose candidates. For most of American history, party leaders picked nominees at conventions and in back rooms. The primary system grew out of Progressive Era reforms and expanded dramatically after the turbulent 1968 Democratic convention, when new party rules pushed states to let voters choose delegates directly.
This guide explains the types of primaries and caucuses, how delegates work, how the presidential calendar is set, and the debate over the system's effects.
States set primary dates in law, but parties penalize states that vote earlier than allowed. Iowa and New Hampshire went first for decades, which critics said gave outsized influence to small, unrepresentative states. For 2024 Democrats moved South Carolina to the front, and in August 2026 the Democratic National Committee approved a 2028 order beginning with South Carolina, Nevada, New Hampshire, New Mexico, Michigan, and Virginia, with Super Tuesday on March 7, 2028. Republicans set their own calendar and have traditionally kept Iowa and New Hampshire first. Many states crowd into early dates, a pattern called front-loading.
A caucus is a meeting of party members who gather in schools, churches, and community halls to discuss candidates and choose delegates. Caucuses reward organized and enthusiastic supporters but require hours of attendance and draw far fewer voters than primaries. After the troubled 2020 Iowa Democratic caucuses, the Democratic Party pushed states toward primaries, and only a handful of states still use caucuses for presidential contests. Republicans in Iowa continue to hold first-in-the-nation caucuses.
Primaries also choose nominees for Congress, governor, and state offices, often with turnout below 20 percent. Because most districts and many states are safe for one party, the primary is frequently the real contest. Critics argue that this rewards candidates who appeal to the most committed partisans and contributes to polarization, and they promote open or top-two primaries and ranked-choice voting. Supporters of closed primaries say parties have the right to choose their own nominees without outsiders, and several states have rejected or repealed reform measures in recent elections.
Primaries and caucuses award delegates to the national conventions, and a candidate needs a majority to win the nomination. Democrats award pledged delegates proportionally among candidates who receive at least 15 percent. Party leaders and elected officials serve as automatic delegates, often called superdelegates, but since 2018 they cannot vote on the first ballot unless no candidate has a majority of pledged delegates. Republican rules vary by state, with some awarding all delegates to the statewide winner. If no one wins a majority, delegates become free to switch on later ballots at a contested convention.
Early nominees were chosen by congressional caucuses and then, from the 1830s, by national conventions controlled by state party leaders. Progressive reformers introduced direct primaries in the early 1900s, but as late as 1968 most convention delegates were not chosen by primary voters. After that year's divisive Democratic convention, the party's McGovern-Fraser Commission rewrote its rules, states added primaries to comply, and Republicans followed. Since 1972 primary voters have effectively chosen both parties' nominees.
In closed primaries, only registered party members may vote. Open primaries let any voter choose either party's ballot, and semi-closed primaries let unaffiliated voters participate. California and Washington use top-two primaries, in which all candidates appear on one ballot and the top two advance regardless of party, and Alaska uses a top-four primary with ranked-choice voting in the general election. The Supreme Court has held that parties have associational rights, striking down California's blanket primary in 2000 but upholding Washington's top-two system in 2008.
See the full presidential cycle in the presidential election, start to finish, learn about candidates outside the two parties in third parties explained, or see the off-year elections in midterm elections explained.