In addition to the pledged delegates, 795 "superdelegates" get to act as free agents and can select whichever candidate they wish for the nomination.
Superdelegates are Democratic members of Congress, governors, former Democratic presidents, as well as state and national party leaders.
They are not bound to their candidate based on the primary or caucus results from their state, and there is no formal process to win them, though campaigns put forth considerable efforts to gain their formal support.
The 795 Democratic superdelegates account for nearly 40 percent of the 2,025 total needed to win the Democratic nomination.
Superdelegates are under no obligation to publicly declare candidate support, which makes counting them an inexact science.
Without overarching party rules, the GOP gives considerable flexibility to state parties in determining how they allocate their delegates. There are three main ways that Republican candidates win delegates: winner-take-all, proportional and by congressional district.
In most a majority of Republican contests, the winner took all state delegates no matter how narrow the margin of victory in the popular vote. In other words, the candidate who won the popular vote won every delegate from the state.
Sen. McCain won several winner-take-all states including Arizona, New York and New Jersey; so, even though his rivals, former governors Mike Huckabee and Mitt Romney won votes, they didn't win any delegates.
But other GOP contests, such as Massachusetts, follow a model similar to that of the Democrats in which delegates are awarded proportionally based on the statewide vote.
Romney won Massachusetts on Super Tuesday but because the state allocates delegates proportionally, McCain won a share of the delegate trove as well.
The remaining states use a combination of winner take all and proportional but divides the state by congressional district.
A candidate wins three delegates from each congressional district based on winning the popular vote within that congressional district.
For example, California, with its 170 delegates at stake, looks like a delegate jackpot.
Given the state's plan for delegate allocation, a narrow win in the statewide vote count does not mean 170 delegates in a candidate's pocket. Instead, they are looking to win in the most congressional districts.
Unlike the Democrats, the Republicans do not have superdelegates.
Some Republican delegates are distinguished as "unpledged" but are not always free agents who are unbound to the state vote on the Democratic side.
In 2007, both parties sanctioned states for moving their nominating contests earlier in the calendar.
The Republican National Committee took away half of the delegate counts for Wyoming, New Hampshire, Michigan, South Carolina and Florida for scheduling their primaries before Feb. 5, 2008.
The Democratic National Committee stripped Michigan and Florida of all of their delegates for holding primaries before Feb. 5.
Superdelegates who pledged their support to Democratic candidates other than Obama or Clinton before they dropped out of the race can switch their support to another candidate or can remain committed to their first pick until the convention in the summer.
ABC News' David Chalian and Talal Alkhatib contributed to this report.