Democratic and Republican candidates compete in state-by-state primary elections and caucuses for delegate support to win their respective party's nomination to run in the general election for president.

A delegate is a party representative, chosen at the state level, to cast a vote in favor of a candidate at the convention. The candidate with the most pledged delegates at the end of the process wins the nomination. The Republicans require that the nominee win a simple majority of 1,191 out of 2,380 delegates. That magic number is 2,025 out of 4,049 for Democrats.

The Democrats and Republicans use different methods to award delegates.

In the Democratic primaries and caucuses, candidates are awarded different numbers of pledged delegates according to the percentage of the vote they earn. For example, in the South Carolina primary:

• Barack Obama won 295,091 votes, or 55 percent of the votes cast, and was awarded 25 delegates.
• Hillary Clinton won 141,128 votes, or 27 percent of the votes cast, and was awarded 12 delegates.
• John Edwards won 93,552 votes, or 18 percent of the votes cast, and was awarded 8 delegates.

The Democrats also have superdelegates, including Democratic National Committee members, U.S. senators, governors and other party leaders. They do not have to declare loyalty to a particular candidate, but some do. Thus, while one candidate may be leading in the race for pledged delegates through the primary races, another candidate may lead on the overall count because of backing by superdelegates.

In the Republican Party, each state chooses either the proportional method or a "winner-take-all" method of awarding delegates. Under the winner-take-all method, the candidate getting the most votes from a state's caucus or primary, gets all of that state's delegates at the national convention. For example, in the Florida primary, John McCain won 693,508 votes, or 36 percent of the vote, and was awarded all 57 delegates.