Hillary Clinton is expected to celebrate becoming the first woman to lead a major American political party following votes in California, New Jersey and four other states.
Mrs Clinton reached the 2,383 delegates needed to become the presumptive Democratic presidential nominee on the eve of Tuesday's voting, according to an Associated Press tally.
Her total is comprised of pledged delegates won in primaries and caucuses, as well as superdelegates - the party officials and officeholders who can back a candidate of their choosing.
Mrs Clinton greeted news of her achievement with a measured response, wary of depressing turnout and eager to save the revelry for a big victory party Tuesday night in Brooklyn.
During a campaign stop in California, Mrs Clinton told a cheering crowd she was on the brink of a "historic, unprecedented moment", but said there was still work to do in her unexpectedly heated primary battle with Vermont Senator Bernie Sanders.
"We're going to fight hard for every single vote," Mrs Clinton declared.
Heading into Tuesday's voting, Clinton has 1,812 pledged delegates and the support of 571 of the 714 superdelegates, according to the AP count.
The AP surveyed the superdelegates repeatedly in the past seven months. While they can change their minds, those counted in Mrs Clinton's tally have unequivocally told the AP they will support her at the party's summer convention.
During a rally on Monday evening in San Francisco, Mr Sanders said a victory in California would give him "enormous momentum" in his bid to push the Democratic primary to a convention fight.
Mr Sanders is urging superdelegates to drop their support for Mrs Clinton before the gathering in Philadelphia, arguing he is a stronger candidate to take on presumptive Republican nominee Donald Trump.
But Sanders has so far been unable to sway the superdelegates, and there were signs on Monday that he was taking stock of his standing in the race. Speaking to reporters, Mr Sanders said he planned to return to Vermont on Wednesday and "assess where we are" following the California results.
The senator's comments came on the heels of a weekend phone call with President Barack Obama, who has stayed out of the Democratic primary to date but is poised to endorse Mrs Clinton as early as this week.
"The president intends certainly through the fall, if not earlier, to engage in this campaign," White House spokesman Josh Earnest said. "That's an opportunity the president relishes."
Mr Obama and Mrs Clinton battled ferociously for the Democratic nomination in 2008. Tuesday marks eight years to the day Mrs Clinton conceded to Mr Obama in an emotional speech where she noted she was unable to "shatter that highest, hardest glass ceiling".
The former secretary of state reflected on breaking that barrier as she made her final swing through California on Monday, and she is expected to do so again on Tuesday night in New York.
"It's really emotional," Mrs Clinton said. "I'm someone who has been very touched and really encouraged by this extraordinary conviction that people have."
Glenda McCarthy, a 64-year-old from San Pedro, California, is among the loyal Clinton supporters who have longed for this milestone moment.
"I've been waiting for this for so long," Ms McCarthy said. "Not just a woman, but a woman who is so strong."
To everyone who's worked so hard, thank you. Let's go win this thing. pic.twitter.com/T6ou2Znh9D— Hillary Clinton (@HillaryClinton) June 7, 2016
Mrs Clinton's victory is broadly decisive. She leads Mr Sanders by more than three million cast votes, by 291 pledged delegates and by 523 superdelegates. She won 29 caucuses and primaries in states and US territories to his 21 victories.
On Tuesday, she received a nod of approval from House minority leader Nancy Pelosi who said she will vote for Mrs Clinton in California's primary, ending months of artful fence-sitting during which Ms Pelosi praised both Mrs Clinton and Mr Sanders.
Mrs Clinton has been eager to move past the protracted primary and fully turn her attention to her general election battle with Mr Trump. She energised Democrats with a blistering speech last week challenging Mr Trump's qualifications for the presidency, reassuring supporters that she is prepared for a bruising campaign against the unpredictable businessman.
Mr Trump vanquished his remaining Republican rivals about a month ago, a stunning achievement for the untested political candidate. Despite his controversial statements about minorities and his vague policy proposals, many Republicans quickly consolidated around his nomination.
But Mr Trump has continued to irritate Republican officials, including with his recent criticism of a US district court judge. Mr Trump has said Judge Gonzalo Curiel cannot be impartial in a legal case involving the businessman because his parents were born in Mexico and Mr Trump wants to build a wall along the border.
House Speaker Paul Ryan said on Tuesday that Mr Trump made the "textbook definition of a racist comment" in saying the American-born judge is not qualified to preside over the case.
Mr Trump also continues to struggle to build out a robust general election campaign staff in battleground states or a national fundraising network, though the real estate mogul insists he can win without the trappings of a traditional campaign.
Mr Trump was also spending Tuesday in New York, with a prime-time event scheduled at his golf resort in Westchester.
New Jersey and California are the biggest prizes up for grabs on Tuesday, with Montana, New Mexico, North Dakota and South Dakota also holding contests. The final Democratic primary will be held next week in the District of Columbia.
Let's finish this primary strong and head into the general election together: https://t.co/3DyAZfFyoi— Hillary Clinton (@HillaryClinton) June 7, 2016