US Primary Elections Explained: How Parties Choose Candidates

A primary election is a contest held before the general election in which registered voters select their party's nominee for a given office. The winner of each party's primary then advances to the general election in November. Primaries exist at every level: presidential, US Senate, US House, governor, state legislature, and many local offices.

The primary system is a distinctly American invention. Most democracies allow party leadership to select candidates through internal party processes. In the US, primary elections — established progressively from the early 1900s onward — opened candidate selection to rank-and-file voters rather than party bosses. The McGovern-Fraser reforms after 1968 extended this democratization to presidential primaries.

The flip side: because primary electorates are smaller, more ideologically engaged, and less representative of the general electorate, candidates who win primaries have sometimes been poorly positioned for general elections. This tension between pleasing the base and winning in November is a central dynamic of every election cycle.

In approximately 11 states — mostly in the South — a candidate must win a majority (50%+1) of the primary vote to advance. If no candidate clears 50%, the top two candidates face a runoff election held several weeks later. States using runoff systems include Georgia, Alabama, Mississippi, Louisiana, South Carolina, and Texas.

Runoffs were originally designed in the South to prevent racially diverse fields from producing nominees with narrow pluralities. Their modern effect is to give party activists and donors a second opportunity to influence nominations, and to potentially elevate candidates who can consolidate support behind a single alternative to the primary frontrunner.

Runoff turnout is typically significantly lower than first-round primary turnout — sometimes 30-50% lower. This means the electorate in runoffs is often dominated by the most motivated, ideologically committed voters, which can produce different outcomes than would occur with a broader turnout.

Presidential primaries allocate delegates to national party conventions. The Democratic and Republican parties have different delegate allocation rules. Democrats use proportional allocation: candidates who reach a 15% threshold in a state or congressional district receive delegates proportional to their share of the vote above the threshold. This means close multi-candidate races can extend deep into the calendar. Republicans have a mix of winner-take-all and proportional rules that vary by state.

Congressional and state-level primaries typically elect a single winner by plurality or majority, depending on state rules. In most states, the candidate with the most votes wins — even with a plurality well below 50%. This means a crowded primary field can produce a nominee who received only 25-30% of the vote, potentially someone who is not the preferred choice of a majority of primary voters.

The 2026 cycle will feature high-profile Senate primaries in Georgia, Michigan, New Hampshire, and Virginia (Democratic) and potentially competitive Republican primaries in Nevada, Maine, and other states where incumbents may face challenges from the right.

Approximately 11 states — mostly in the South — use majority-threshold runoff rules. California and Washington use top-two for all offices; Alaska uses top-four with ranked-choice voting for state and federal races.