Story in numbers: Election of the US President

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A series of elections are held in every state, starting in February, which determine who becomes each party's official presidential candidate. The winner of each collects a number of 'delegates' - members with the power to vote for that candidate at the party conventions in July, where candidates are formally confirmed. The more state contests a candidate wins, the more delegates will be pledged to support them at the convention. As President Barack Obama cannot run again, both parties are holding competitive primaries this year.