During the primaries, presidential candidates must win the support of delegates—individuals who pledge to support them during their party's national nominating convention. These delegates are usually made up of local party leaders and activists. 

Since the disastrous 1968 Democratic National Convention—when Hubert Humphrey became the nominee despite failing to win a single primary—most delegates are bound to follow the popular will and support the winner of their state’s primary or caucus.

On the Democratic side, of the 4,700 delegates each election season, around 15 percent are so-called superdelegates, who can support any candidate they choose and can switch their support at any time, right up to the actual nomination. Superdelegates are major elected officials (including senators and members of the House of Representatives), notable party members (current or former presidents and vice presidents) and some members of the Democratic National Committee (DNC)—put simply, the Democratic elite. 

After 1968, the Democrats produced some relatively weak nominees: George McGovern carried only one state, plus the District of Columbia, in his loss to Richard Nixon in 1972, and in 1980 Jimmy Carter lost reelection to Ronald Reagan by only a slightly less humiliating margin. In the wake of such losses, leading Democrats decided to reform the nominating process so that the party’s elite members could play more of a role in selecting nominees, and choose candidates they believed would fare better in the general election.

Theoretically, superdelegates could change the results of a nominating process, but in practice they rarely have. Since the reforms were adopted in 1982, all superdelegates have followed the results of the popular vote in the primaries at the convention. The only time superdelegates directly exerted their influence was in 1984, when they pushed Walter Mondale to the nomination after he won the pledged delegate count by too narrow a margin to secure victory. (Mondale would win only one state plus D.C. in his loss to Reagan.)

The Republican Party doesn’t use superdelegates in the same way as the Democrats. In addition to a certain number of delegates based on its size, each state has three delegates from the Republican National Committee (RNC) to represent it at the national convention. In the past, these RNC delegates (which represent less than 7 percent of the party’s total delegates in 2016) could be "unpledged," but in 2012 the committee mandated that they were bound to reflect the will of voters in their states.