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What is the Difference Between a Primary and a Caucus?

In Brief

In the United States, voters choose their party’s presidential nominee through state primaries or caucuses. Dating back to the early 19th century, caucuses typically require more commitment from voters and attract more engaged voters. Primaries later emerged as a way to give voters more influence over the nominating process relative to high-ranking party officials. Some states continue to hold caucuses, giving party officials and activists greater control over the process.

Overview: Primaries and Caucuses

Depending on the state and political party, voters in the United States participate in presidential nominating contests through either primaries or caucuses.

  • Primaries are similar to general elections. Voters go to a polling place at any point over the course of the primary election day and cast a secret ballot. Alternatively, voters cast mail-in ballots.
  • Caucuses provide forums for passionate supporters to sway other voters. Some caucuses ask voters to publically declare their vote, while in others, voters cast a secret ballot after hearing speeches given on behalf of candidates. Caucuses are held on weekday evenings or weekends, and they typically last about two hours.

The pool of voters eligible to vote in a party nominating contest differs depending on the rules of each state and party. In closed primaries and caucuses, only registered members of the party can vote, while in open primaries and caucuses, all registered voters can vote in any party’s contest. In semi-closed primaries and caucuses, unaffiliated or independent voters can vote in any party’s contest, but voters registered with a party can vote only in that party’s nominating contest.

Both major parties use a combination of open and closed primaries and caucuses to select their nominee, but they parties may hold different types of nominating contest even in the same state. For example, in 2016, Idaho Republicans held a closed primary while Democrats held an open caucus.

Why are there two forms?

Caucuses date back to the early 19th century as a way for high-ranking party members to select delegates for state and national conventions. Meanwhile, the modern presidential primary began in the 1910s as a Progressive Era reform to expand the influence of typical voters. However, until the 1970s, voters had little impact on the ultimate selection of a presidential nominee. Most delegates at the national party convention represented not primary voters, but the high-ranking party officials who attended caucuses.

At the 1968 Democratic Nominating Convention, convention-goers selected  party elites’ choice over the choice of most voters, resulting in riots.  In response, Democrats adopted reforms compelling more states to adopt primaries instead of caucuses, thereby providing voters with greater influence over the party’s ultimate nominee. Though some states continued to hold caucuses, they were opened to the general public. As Democrat-led reforms passed state legislatures, Republicans often adopted similar reforms for their own party. In 1968, voters selected only one-third of national convention delegates, but by 1972, they selected two-thirds of delegates.

How do caucus voters differ from primary voters?

Because caucuses may take hours and require more active voter participation, their turnout rates are typically much lower than primaries’. Among the first 22 states that voted in 2016, just 11.3% of registered voters in a caucus state voted in a nominating contest, compared to 36.1% in primary states.

Caucuses also tend to attract a different pool of voters than primaries. Research suggests that caucus-goers are more politically and civically engaged than primary voters, though they are not necessarily more ideologically extreme. Consequently, candidates with very committed supporters typically fare significantly better in caucus states than in primary states. For example, in the 2008 election cycle, Barack Obama performed 13% better on average in caucuses than he did in primaries.

Because caucuses occur at a designated time and place, they also risk excluding certain groups of voters such as the elderly and disabled, those serving in the military, and those who have work or religious obligations at that time. Some states, like Washington, have taken steps to include these groups, by allowing qualifying voters to submit a form remotely instead of attending.

Which states have primaries and which have caucuses?

During the 2016 presidential nomination season, 14 states held caucuses for one or both major parties’ nominating contests, while the remaining states held primaries.


State parties balance competing priorities of cost and control when deciding to hold a caucus or a primary. State governments control and fully fund primaries. State parties have more control over caucuses – they can set factors like the date and voter eligibility – but they must also fund them.

While state parties choose the type of contest, national parties maintain some control over the overall voting schedule. For example, because they believe small states can provide a launching pad for skilled candidates with limited financial resources, the national parties allow Iowa (the first caucus) and New Hampshire (the first primary) to maintain their influential early positions in the nominating process.


  • While primaries are similar to the general election voting process, caucuses offer a space for supporters to advocate for their candidate.
  • Caucuses and primaries draw different pools of voters, with caucuses tending to draw smaller groups of more politically engaged individuals.
  • The modern presidential primary emerged as a way to elevate the role of voters relative to high-ranking party officials.
  • State parties balance cost and control in deciding to hold a primary and caucus while national parties control the voting schedule.


Works Cited [+ Expand]

FairVote. “Primaries.” February 2016.

Goldford, Dennis, Dougherty, Ann, Mangino, Stephanie, and Claxton, Donny. Slate. “What’s a caucus?” June 29, 1999.

Greenberg, David. Slate. “My Vote Means Nothing.” June 11, 2007.

Halloran, Liz. NPR. “Why Bother With Caucuses?” February 7, 2012.

Hersh, Eitan. “Primary Voters Versus Caucus Goers and the Peripheral Motivations of Political Participation.” Political Behavior. June 17, 2011.

Masket, Seth. Pacific Standard. “To Primary or to Caucus?” April 27, 2015.

Piroth, Scott. “Selecting Presidential Nominees: The Evolution of the Current System and Prospects for Reform.” Social Education. September 2000.

Putnam, Josh. The Washington Post. “Everything you need to know about how the presidential primary works.” May 12, 2015.

Sabato, Larry J. Rasmussen Reports. “Primaries Versus Caucuses: The Score So Far in 2016.” April 21, 2016.

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