Hillary Clinton faces the last major contest of the primary campaign on Tuesday having already been declared the Democratic presidential nominee, making her the first woman in history to lead a major party bid for the White House.

The declaration that Clinton had won the support of the 2,383 delegates needed to clinch the nomination came from the Associated Press late on Monday, before voting was due to commence in primaries in California and five other states.

The legitimacy of AP’s declaration, which was announced 24 hours earlier than her campaign expected, was immediately called into question by Clinton’s rival, Bernie Sanders.

The Vermont senator’s campaign issued a defiant statement that condemned the media’s “rush to judgment” and signalled that the Vermont senator was willing, if possible, to contest the nomination at the Democratic National Convention in July.

However, as voters headed to the polls in California, New Jersey, Montana, North Dakota, South Dakota and New Mexico, it was clear that the mathematics were squarely on the side of the former secretary of state.

The unexpected and somewhat anti-climactic twist in the race appeared to surprise the Clinton campaign, which has not altered its plan and is waiting until voting concludes on Tuesday before declaring her the Democratic nominee-in-waiting at a victory party in New York.

Clinton made reference to the AP declaration during a campaign event in Long Beach, California, on Monday night. “I got to tell you, according to the news, we are on the brink of a historic, historic, unprecedented moment, but we still have work to do, don’t we?” she said.

On Tuesday Clinton secured the endorsement of House Democratic leader Nancy Pelosi of California and, according to US media reports, aides to Barack Obama are in discussion with her campaign with a view to the president formally backing her soon. He is understood to have called Sanders on Sunday to inform him.

White House spokesman Josh Earnest said: “The president intends certainly through the fall, if not earlier, to engage in this campaign. That’s an opportunity the president relishes.”

Clinton’s candidacy, years in the making, will cap a long and bruising campaign against Sanders, a self-described socialist who has electrified the progressive wing of the Democratic party and pulled its frontrunner to the left.

Her graduation to presumptive nominee will also mark the start of a momentous general election campaign against the Republican nominee, Donald Trump.

Clinton gave a foretaste of the type of campaign she plans to wage against the real estate mogul last week when she used a speech in San Diego to brand her adversary too dangerous and unstable to be entrusted with nuclear codes and warning of economic crisis if he were to reach the White House.

“Donald Trump’s ideas aren’t just different, they’re dangerously incoherent. They’re not even really ideas, just a series of bizarre rants, personal feuds and outright lies,” she said in that speech, widely agreed to be one of her best of the campaign. “He is not just unprepared. He is temperamentally unfit to hold an office that requires knowledge, stability and immense responsibility.”

At a fundraising concert in Los Angeles on Monday where celebrity supporters included Stevie Wonder, Ricky Martin, Cher, Magic Johnson and Christina Aguilera, Clinton told the crowd: “We’re going to come out of the primary even stronger to take on Donald Trump. Enough with the fear, enough with the anger, enough with the bigotry, enough with the bullying!”

Her supporters argue she has unparalleled qualifications for the job after a lifetime in public service in which she has served as first lady, New York senator and secretary of state under Barack Obama.

The US president, who defeated Clinton’s first bid for the Democratic nomination in 2008, is widely expected to endorse Clinton in the coming days.

The sense that the nomination was within Clinton’s grasp had been growing in recent days and the candidate has been looking increasingly relaxed and confident on the campaign trail.

Earlier on Monday, in an exchange with reporters in Compton, Clinton made clear she believed Sanders should withdraw from the race after Tuesday’s vote, pointing out it would be “eight years to the day” since she withdrew and endorsed then-senator Obama.

Unusually for Clinton, who has carefully avoided appearing to take the nomination for granted, she also conceded she was “very touched” by the belief among her supporters that she was on the verge of making history.

“My supporters are passionate. They are committed. They have voted for me in great numbers across our country for many reasons,” she said.

“But among those reasons is their belief that having a woman president will make a great statement, a historic statement about what kind of country we are, what we stand for. It’s really emotional.”

However, Clinton’s readiness for the looming general election battle stills rests, in part, on the outcome of Tuesday’s primary in California, a large and diverse state that she had been expected to win easily until just a few weeks ago.

Clinton and her husband, former president Bill Clinton, have campaigned tirelessly in the state in recent days after polls showed her formidable lead in the polls shrink in the face of a stiff challenge from Sanders.

The senator had hoped to use an upset in California to shift momentum in the race and convince superdelegates to switch sides.

Conversely, a defeat for Sanders in California, which could potentially mean his rival amounting sufficient pledged delegates to seal the nomination without the help of superdelegates, would fundamentally undermine his case for remaining in the race until July.

Appearing before thousands of supporters in front of a fog-shrouded Golden Gate Bridge late on Monday, Sanders implored supporters to turn out for a contest he described as “the most important primary that we’ve had in the entire Democratic nomination process”.

He repeated his argument that he is consistently performing better against Trump in the polls than Clinton and stands a better chance of keeping him out of the White House. The senator made no mention of the reports declaring Clinton the nominee, but the news had by then percolated through the crowd.

Some supporters began trickling out of the rally before it had concluded while others sniped at reporters over what they complained was biased media coverage and a premature and undemocratic declaration of Clinton as the victor.

AP said its announcement was based on “a burst of last-minute support from superdelegates” – the party officials who get a vote at the national convention that is not bound by the results of elections.

Clinton appeared to be on the very cusp of amassing the number of delegates to win the nomination after convincing wins in the US Virgin Islands and Puerto Rico by the end of the weekend.

By then Clinton had 1,812 pledged delegates, compared with Sanders’ 1,521. When those were added to her overwhelming support from superdelegates the frontrunner appeared to be just shy of the target.

But the declaration, by AP’s count, that Clinton had actually met the target appeared to catch both campaigns off guard, upending carefully choreographed plans to react to a denouement not expected until Tuesday.

Sanders’ spokesman, Michael Briggs, immediately released a statement accusing the media of “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”.

“Secretary Clinton does not have and will not have the requisite number of pledged delegates to secure the nomination,” he said. “She will be dependent on superdelegates who do not vote until 25 July and who can change their minds between now and then.”

Even Clinton’s campaign appeared to believe the declaration was premature. “We can’t say the primary is over,” Bill Clinton told reporters at a rally in San Francisco. “Let people vote. Let them have their say.” Other Clinton campaign officials indicated that, while the news reports were welcome, they did not plan to declare victory in the overall race until Tuesday.

Clinton spoke only briefly at the concert-rally in Hollywood. “It is not an overstatement for me to say that we have a really important election ahead of us now,” she said, smiling broadly.