Mrs Hillary Clinton become the first female nominee for a major US political party, after vote counts by the super-delegates puts her on 2,383 – the number needed to make her the presumptive nominee.
But rival Bernie Sanders said Mrs Clinton had not won as she was dependent on superdelegates who could not vote until July’s party convention.
Mrs Clinton reached the threshold with a big win in Puerto Rico and a burst of last-minute support from superdelegates, AP said late on Monday night.
Superdelegates are party insiders who can pledge their support for a candidate ahead of the convention but do not formally vote for them until the convention itself.
AP’s announcement came ahead of Democratic primaries on Tuesday in California, Montana, New Mexico, North Dakota, South Dakota, and New Jersey. Polling stations opened at 06:00 local time (10:00 GMT) in New Jersey. The last ones to close will be in California at 03:00 GMT Wednesday.
It has taken a long 227 years to get even this far.
George Washington was elected president of a newly independent America in 1789. Forty-three men later (42 of them white) Hillary Clinton makes history today by being the first female nominee for the White House.
The lack of exuberance may come from the fact that this has all been going on for so long. We’ve really been reworking a version of the “first viable female candidate for the presidency” story since 20 January 2007, the first time Hillary Clinton declared her candidacy for the White House.
A woman president would be new, Hillary Clinton is not new to governance.
At an appearance in Long Beach, California, shortly after the news broke, Mrs Clinton said: “We are on the brink of a historic and unprecedented moment but we still have work to do.
“We have six elections tomorrow and we’re going to fight hard for every single vote, especially right here in California.”
Senator Sanders has vowed to stay in the race until the convention, and his campaign team said the Vermont senator would attempt to win back superdelegates.
His spokesman Michael Briggs said it was too early to call the contest.
“It is unfortunate that the media, in a rush to judgement, are ignoring the Democratic National Committee’s clear statement that it is wrong to count the votes of superdelegates before they actually vote at the convention this summer,” he said.
Mrs Clinton, a former secretary of state, New York senator and First Lady, leads Mr Sanders by three million votes, 291 pledged delegates and 523 superdelegates, according to AP’s count.
She has won 29 caucuses and primaries to his 21 victories – and an estimated 2.9 million more voters have backed her during the nominating process.
That gives her a significantly greater lead over Mr Sanders than Barack Obama had over her in 2008 – he led by 131 pledged delegates and 105 superdelegates at the point he clinched the nomination.
“This is a subject of great consternation to rival Sen. Bernie Sanders, who… has repeatedly suggested that media outlets who point this out are irresponsibly abdicating their duty to their audiences. But it’s not a complicated issue. If you accept that Donald Trump has clinched the Republican nomination, you should similarly accept that Clinton will seal her party’s nomination on Tuesday.” said Philip Bump in the Washington Post