The New Hampshire primary is the first in a series of nationwide political party primary elections held in the United States every four years, as part of the process of choosing the Democratic and Republican nominees for the presidential elections to be held the subsequent November. Held in the small New England state of New Hampshire, it traditionally marks the opening of the quadrennial U.S. presidential election, although that status was threatened in 2007, as both the Republican and Democratic National Committees moved to give more populous states a bigger influence in the presidential race. This is partly because New Hampshire has so little impact, in terms of delegates, when compared to Super Tuesday. Its real impact comes from the media coverage and momentum that a candidate can attain from a decisive or better-than-expected result in the New Hampshire primary. Several states also sought to move up the dates of their 2008 primaries in order to have more influence and dilute the power of the New Hampshire primary.

Originally held in March, its date has been moved up repeatedly to maintain New Hampshire's first-in-the-nation status in the face of ever-earlier primaries in other states. New Hampshire law stipulates that the New Hampshire primary will be the first primary held in the United States, and although the Iowa caucuses are held before the New Hampshire primary, because the Iowa event are caucuses, for the purposes of New Hampshire law it is not counted as a primary. The 2008 primary was held on January 8. Since 1952, the primary has been a major testing ground for candidates for both the Republican and Democratic nominations. Candidates who do poorly frequently drop out, while lesser-known, underfunded candidates who do well in New Hampshire suddenly become serious contenders, garnering large amounts of media attention and campaign funding.

It is not a closed primary, in which votes can be cast in a party primary only by people registered with that party. Undeclared voters — those not registered with any party — can vote in either party primary. The term the state of N.H. uses for voters not affiliating with a party is "undeclared" See the section entitled "Political Parties" in the source. However, it does not meet a common definition of an open primary, because people registered as Republican or Democrat on voting day cannot cast ballots in the primary of the other party.