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The Presidential Primary System, Explained

Unpacking a confusing convoluted process that determines our choices in November

Carlo Macomber
Research Assistant
January 3, 2024

The 2024 Republican presidential primary kicks off Monday with the Iowa Caucuses. At the moment, there are two main challengers to former President Donald Trump: Florida Gov. Ron DeSantis and former South Carolina Gov. Nikki Haley. While sitting presidents are rarely ever challenged for renomination, a few lesser-known Democrats have jumped into the fray to provide a challenge, if only a nominal one, to President Joe Biden.

Despite polls showing most Americans do not want either Trump or Biden to run for president at all, nearly all political observers believe we are headed toward a rematch of the 2020 election. Why? One major reason: The presidential primary system.

The current presidential primary process is both long and convoluted. Over two centuries, it has taken different forms, ranging from congressional caucuses choosing nominees behind closed doors, to party bosses negotiating in smoked-fill rooms (and often ignoring the outcomes of non-binding primaries). Today, it’s a primary-driven process, but one that is significantly more complicated and confusing than the direct primary process used in congressional and state elections.

Primary rules are also considerably different depending on which state voters live in and what political party they belong to. There are even states that still do not hold primaries at all, opting for in-person caucuses instead. On top of all of this, there is also debate every cycle about the order of presidential primaries and caucuses. The states that vote first hold extra influence, which historically comes with being an “early primary state.”

With so many moving parts and confusing rules, this explainer unpacks key components in the U.S. presidential primary process, including the calendar, participation rules, and how votes cast translate into winners. The structure of the system, and its underlying rules, often has an outsized impact on which candidates are nominated.

The Calendar

One quirk of the presidential primary process is that while it drags on for several months, the states that vote first hold outsized influence. In general, the candidates who perform well in the first handful of contests are the ones who are viable over the long run. In 2024, the party nomination processes will run from mid-January until early-June. Typically, there is one Tuesday in early March, known as “Super Tuesday,” on which more than a dozen states hold their presidential primaries. By Super Tuesday, if not before, the likely nominee for each party is usually quite clear.

Traditionally, Iowa and New Hampshire are the first states to vote. However, in February 2023, the Democratic National Committee approved a plan to alter the primary calendar. Under the new calendar, the first four states to hold Democratic presidential primaries will be, in order: South Carolina, New Hampshire, Nevada, and Michigan.

Notably, Iowa is no longer among the early states. The Democratic Party decided to move its caucus later in the calendar following the state’s significant issues with reporting its 2020 caucus results, and because the state’s population is not reflective of the nation’s demographic or ideological diversity. In October 2023, the national and state parties agreed to a plan where Iowa will hold its caucus in mid-January, but the state will not report the results until Super Tuesday.

The rules that govern the presidential primary process exist through a combination of party by-laws and state statutes. For example, the DNC passed the above plan for their primary calendar, but each state can set, by law, the date of their presidential primary or caucus. If New Hampshire, for example, still wishes to hold its primary on an earlier date than South Carolina’s primary (which it does), the state could do so. At the same time, though, the DNC (and RNC) ultimately controls the nomination and convention process, including the allocation of delegates. In order to dissuade states from holding their primaries or caucuses on days that do not align with their calendar, the DNC passed a rule that would take away half of the convention delegates of any state that does not follow the party’s order.

In sum, the timing of primaries is always subject to the whims of parties and state legislators. For the process to run smoothly, there is at least some dependency on agreement between the by-laws of both national party committees, state party committees, and state legislatures.

Who Can Participate in Each State?

As described in “Types of Primary Systems, Explained,” which voters are eligible to participate in state and congressional primaries depends on state rules. The same is true in the presidential primary system. Another weird quirk of America’s primary system is that several states have different rules regarding who can vote in presidential primaries than they do for their state and congressional primaries.

For presidential contests, each of the 50 states fits into one of the three partisan primary categories (as there are no nonpartisan presidential primaries). (U.S. territories also participate in the presidential nominating process.)

(In total, seven of the 50 states will hold caucuses on the Republican side, while just three will do so on the Democratic side.)

Delegate Allocation

The modern presidential primary process is technically indirect: Voters do not directly nominate candidates. Instead, their votes for a given candidate translate into a number of delegates who are bound to support that candidate at a national party convention. A candidate must win a majority of delegates at the convention to capture the nomination.

As a result, the process by which votes turn into convention delegates is crucial to understanding how each major party nominates its candidate. The two parties use very different systems to allocate their delegates, which present candidates with contrasting incentives. In every state nominating contest, the Democratic Party uses a proportional representation system to allocate delegates, while the Republican Party uses a variety of delegate allocation methods with a strong bias in favor of “winner-take-all” systems.

There are, however, some similarities between the two parties’ systems that set the basis for the process of delegate allocation.

  • First, each state has a predetermined number of delegates that are allocated among candidates based on the outcomes of each state’s primary or caucus. While the states have different numbers of delegates on the Republican and Democratic sides, their total is determined by a formula established by each political party committee.
  • Second, the delegates in each state are further categorized as either “statewide” or “congressional district” delegates, indicating at which level they are awarded.

Beyond these similarities, the two parties’ allocation methods are quite different.

Democratic Delegate Allocation

The Democrats’ proportional system of delegate allocation may initially appear simple and straightforward, but there are several complicating factors. On the surface, candidates receive a proportion of delegates that is equivalent to their vote share, but this formula is applied at both the statewide and congressional district levels. A candidate who receives 50% of the statewide vote should win half of the state’s statewide delegates, while a candidate who receives 60% of the vote in a given congressional district should win 60% of that district’s delegates. 

Further complicating the process is the minimum threshold necessary to win delegates. Per party rules, candidates are only eligible to receive a share of delegates if they earn at least 15% of the vote. This rule applies to both statewide and congressional district delegates. As a result, a candidate who receives 14% of the statewide vote and a candidate who receives 1% of the statewide vote receive the same number of statewide delegates: 0. However, candidates who receive less than 15% of the statewide vote are still eligible to earn congressional district delegates if they reach the 15% threshold in any district. This happened in Iowa in 2020 when Sen. Amy Klobuchar received just 12.2% of the statewide vote but was still awarded one total delegate as a result of reaching the 15% threshold in one congressional district.

The 15% threshold can significantly impact the delegate math in a given state. In allocating delegates, votes cast for candidates who do not reach the threshold are essentially discarded. This means that only votes cast for candidates who meet the threshold matter in the delegate allocation process. As a result, when a large percentage of votes are cast for candidates who do not meet the threshold, the candidates who did reach 15% end up receiving a larger share of delegates than their vote shares would otherwise indicate.

Republican Delegate Allocation

The Republican Party uses a variety of delegate allocation methods throughout the country. There are five main categories of delegate allocation that states use. The table below displays these different methods as well as the share of 2024 Republican delegates that will be allocated via each method.

2024 Republican Delegate Allocation

Click on the sections of the pie chart below to learn more about how delegates are allocated under each method.


With different rules set by both individual states and party committees as well as significant variation in the process every four years, it should not be a surprise that the presidential primary process is mystifying to most voters. Each cycle, voters must (1) figure out when their state’s primary is, (2) determine if they are eligible to participate (or what they have to do to register with a party to do so), and, especially if they live in a “late primary state,” (3) decide if the contest is still competitive, and if it is worth their time to participate. Thus, it should perhaps be expected that presidential primary turnout is far lower than general election turnout: In 2020, for instance, roughly 67% of the nation’s voting eligible population participated in the general election compared to about 24% who participated in presidential primaries and caucuses.

The presidential primary process is, nevertheless, extremely important, as it determines the two most viable candidates for our nation’s highest office. But, as the second part of this mini-series details, presidential primaries are rife with problems: Their complicated and exclusionary rules lead to unrepresentative outcomes, producing nominees that most general election voters do not support.