The United States presidential nominating convention is held every four years by political parties fielding nominees in the upcoming U.S. presidential election.

The convention's purpose is to select a party's nominee for president, adopt a statement of party principles and goals (called the platform) and adopt rules for the party's activities.

The convention generally refers to the two major parties events: The Democratic National Convention and the Republican National Convention. Minor parties including the Green Party, Socialist Party USA, Libertarian Party, Constitution Party, and Reform Party USA, also select their nominees by convention.

America's first political convention took place in Hartford, Conn., March 1766. The Sons of Liberty organized the meeting to challenge incumbent Gov. Thomas Fitch, nominating William Pitkin for governor and Jonathan Trumbull for deputy governor.

Throughout the 19th century, public participation was minimal as state party leaders controlled delegate selection. In the early 1900s, reform-minded politicians started demanding that the nomination process become more democratic.

In 1912, former President Theodore Roosevelt won nine primaries, and more than 40 percent of the delegates, compared to challenger and incumbent William Howard Taft's one. Yet Taft received the nomination.

To avoid future boss-dominated politics, more states started to adopt primaries even though they were not considered an important part of the nomination process. State primaries demonstrated a candidate's popularity, but it wouldn't secure a candidate's nomination. It wouldn't be until after 1968 that primaries became relevant.

Conventions were often heated affairs and there was none more tumultuous than the 1968 Democratic National Convention. In the spring of that year, both Robert Kennedy and Martin Luther King Jr. and were assassinated. Convention delegates were sharply divided over the Vietnam War, and protesters marched in the streets of Chicago calling for an end to the war. The clashes between police and protesters were broadcast on national television.

That same year, Sen. Eugene McCarthy challenged Lyndon Johnson in the New Hampshire primary; McCarthy received more than 42 percent of the vote. Just four days later, Robert Kennedy declared his candidacy.

Both McCarthy and Kennedy were opposed to the war and with public opposition to the war mounting; Johnson bowed out of the election. His vice president, Hubert Humphrey, eventually announced his intention to run.

McCarthy had strong grassroots support, but Humphrey's support from most of the state party bosses had delegates demanding reforms to the convention process. As a result, the McGovern-Fraser Commission was created to examine current rules and make recommendations to broaden and encourage participation.

Chaired by Sen. George McGovern and Rep. Donald Fraser, the commission recommended that registered Democratic voters have "the maximum feasible opportunity to participate in the delegate selection process," and recommended that women, minorities and young people be represented based on the percentage of their population within each state.

After the commission's recommendations, the Republican National Committee created the Delegates and Organization Committee. They came up with their own recommendations and they included: Ensuring there will be an equal number of both male and female delegates in each state convention; setting up a quota for delegates under 25 years old; and keeping a mandatory level of minority membership on the national conventions' four standing committees.