<P> In the modern U.S. presidential election process, voters participating in the presidential primaries are actually helping to select many of the delegates to these conventions, who then in turn are pledged to help a specific presidential candidate get nominated . Other delegates to these conventions include political party members who are seated automatically, and are called "unpledged delegates" because they can choose for themselves for which candidate they vote . </P> <P> Generally, usage of "presidential campaign nominating convention" refers to the two major parties' quadrennial events: the Democratic National Convention and the Republican National Convention . Some minor parties also select their nominees by convention, including the Green Party, the Socialist Party USA, the Libertarian Party, the Constitution Party, and the Reform Party USA . </P> <P> The convention cycle begins with the Call to Convention . Usually issued about 18 months in advance, the Call is an invitation from the national party to the state and territory parties to convene to select a presidential nominee . It also sets out the number of delegates to be awarded to each, as well as the rules for the nomination process . The conventions are usually scheduled for four days of business, with the exception of the 1972 Republican and 2012 Democratic conventions, which were three days each . (The 2008 and 2012 Republican conventions were also three days each, but in each case was shortened from the scheduled four days due to weather issues). </P> <P> There is no rule dictating the order of the conventions, but since 1956 the incumbent party has held its convention second . Between 1864 and 1952, the Democrats went second every year (except for 1888). In 1956, when Republican Dwight D. Eisenhower was the incumbent, the Democrats went first, and the party out of power has gone first ever since . (Between 1936 and 1952, the Democrats were the incumbent party and went second, but it is unclear whether they went second because they held the White House or because they had always gone second .) Since 1952, all major party conventions have been held in the months of July, August or (for the first time in 2004), early September . (Election laws in some states would likely prevent conventions from moving into mid-September). Between the middle of the 20th century and 2004, the two major party conventions were primarily scheduled about one month apart, often with the Summer Olympics in between so they did not have to compete for viewers . In 1996, both were held in August to accommodate the Atlanta Olympics in July, the last Summer Olympics to date to be played in the U.S. In 2000, both conventions preceded the Sydney Olympics in late September . </P>

Who chooses the presidential candidate at the convention