Candidates wishing to run for the presidency must enter primary elections held by each party in states throughout the United States. The primary season begins in January of the year in which a presidential election is held. Traditionally, it has begun in Iowa, in which a state caucus select delegates to the national convention. Following Iowa is the New Hampshire primary. Even though these are small states and have few votes, the momentum gained or lost by a victory or defeat in these early states is critical in obtaining money for the rest of the campaign.

Following these states are often blocks of primaries that are held on certain dates, such as "Super Tuesday," when many states hold their nominations. Candidates may choose not to participate in all the primaries, but they must constantly accumulate delegates for the convention and raise funds.

There are two basic types of primaries: open and closed. In open primaries, all registered voters may vote to nominate a candidate. Most primaries, however, are closed primaries, in which only registered members of the party can vote to nominate the party's candidates. The two major parties, Democratic and Republican, nominate their presidential candidates at a national nominating convention. Until recently, the majority of the delegates to the national convention had been appointed by local party officials. Today, the overwhelming majority are elected through the primary system, thus making the system more democratic, while at the same time eliminating most of the drama and purpose of the convention. Since today's primaries determine the makeup of the convention floor, the nominations of the presidential candidates are a foregone conclusion. Conventions have turned into "made-for-TV" specials to advertise the parties' presidential choices. Nominees for Vice President are also officially selected at conventions, but the Presidential candidate is actually the person who chooses the running mate.