Hillary Clinton has clinched the Democratic nomination for US president after reaching the required number of delegates, an AP tally suggests.
The count puts Clinton on 2,383 - the number needed to make her the presumptive nominee.
She will become the first female nominee for a major US political party.
But rival Bernie Sanders said Clinton had not won as she was dependent on superdelegates who could not vote until July's party convention.
Clinton reached the threshold with a big win in Puerto Rico and a burst of last-minute support from superdelegates, AP said.
At an appearance in Long Beach, California, shortly after the news broke, she 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 gonna fight hard for every single vote, especially right here in California."
Voters will go the polls for Democratic primaries tomorrow in California, Montana, New Mexico, North Dakota, South Dakota, and New Jersey.
Superdelegates, who are party insiders, can pledge their support for a candidate ahead of the convention but do not formally vote for them until the convention itself.
The nominee for either party is not officially named until the parties' respective conventions.
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 who have pledged their support to Clinton.
His spokesman Michael Briggs said it was too early to call the Democratic 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.
"Our job from now until the convention is to convince those superdelegates that Bernie is by far the strongest candidate against Donald Trump."
Clinton, a former secretary of state, New York senator and first lady, leads Sanders by three million votes, 291 pledged delegates and 523 superdelegates, according to AP's count.
She 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 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.
Speaking at a rally in San Francisco, Sanders promised supporters he would win in California: "As of today, we have won primaries and caucuses in twenty states across this country.
"And tomorrow, in the most important primary in the whole Democratic nominating process, we're going to win here in California."