Barack Obama has declared himself "the Democratic nominee for president of the United States".He was speaking to a cheering crowd on the last day of the primary season, after receiving the support of enough delegates to clinch the nomination.
Of the states that voted, Montana was won by Mr Obama and South Dakota by his rival Hillary Clinton.
If confirmed, Mr Obama would be the first black candidate to represent a major party in a US presidential poll.
In her own speech to supporters, Mrs Clinton refused to concede and said she would make a final decision later.
BBC North America editor Justin Webb says a fabulously well-planned and executed campaign helped give him the edge over his rival.
Role for Clinton?
Mr Obama's speech was delivered in St Paul, Minnesota, where Republicans are set to hold their presidential nominating convention.
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