That is because in the months before the conventions, U.S. states and territories hold primary elections in which registered voters cast ballots for the candidates they would like to nominate. Republican voters choose from among the Republican contenders, and Democrats do the same for candidates in their own party. In some places, voters who are registered as independents, and belong to neither party, can vote for either a Democrat or a Republican but not both.
In these primary elections, the commitments of convention delegates are at stake. These delegates are people who will travel to the parties’ gatherings as representatives in the formal nomination votes.
In New Hampshire, for example, several hundred thousand people participated in primary elections in February to determine which candidates the state’s small number of delegates will support in July at the Republican National Convention in Cleveland, Ohio, and the Democratic National Convention in Philadelphia.
Over the course of many primary elections across the United States, one candidate in each party usually wins the majority of available delegates. At that point, his or her nomination is essentially a certainty. Even before a candidate wins a majority, rivals often withdraw from the race, as victory becomes increasingly unlikely for them, helping to clear the path for the party’s strongest candidate. The Democratic Party’s primary looks likely to follow the normal course this year.
Several months before the Republican National Convention in 2012, for instance, Republican Mitt Romney earned the title of “presumptive” nominee, because he won delegates through primary elections and because his competitors dropped out.
The Republican Party did not formally nominate Romney for president until the delegates voted at the convention, held in late August that year, but by then it had been clear for a long time that he would be the choice.
It is possible, though — especially among today’s Republican candidates — that no candidate will earn a majority of the delegates before the 2016 convention. A majority is required to win the nomination. So although Donald Trump consistently leads the Republican field in national polls, with his support generally between 30 percent and 40 percent this year, that level of backing among delegates would not be enough to secure the nomination.
If no candidate can attain a majority of delegates by June, when the final primary elections are held, then the Republican Party would hold what is known as a “brokered” convention. Delegates from all of the states and territories would first participate in a ceremonial vote, casting ballots in accordance with the results of the primary elections. And, of course, there wouldn't be a majority winner.
At that point, things would get complicated and potentially dramatic — quickly. Voting obligations vary by state, but after the initial round of balloting, many delegates would no longer have to honor their prior commitments. In this second round of balloting, for example, a delegate who had been pledged to support Ben Carson, who has dropped out of the race, could vote instead for Ted Cruz.
The party would hold round after round of balloting until one candidate received a majority of the votes. Over the course of these votes, there would be lots of dealmaking and chaos. Basically, in a brokered convention, the nomination would come down to which candidate could most effectively convince delegates to abandon their original allegiances and vote for him instead — until one of them gets a majority.