Hillary Clinton captured the Democratic White House nomination hours before Tuesday's last major primaries of 2016, according to U.S. delegate counts, taking a monumental step toward becoming America's first female commander-in-chief.

Passing the milestone of 2,383 delegates secures Clinton's status as the presumptive Democratic nominee, and marks a dramatic political resurgence for a highly experienced but controversial candidate who lost to Barack Obama in their 2008 battle to be the Democratic standard-bearer.

This time the 68-year-old former secretary of state survived an extraordinarily strong grassroots campaign by her party rival Bernie Sanders and is set to go head-to-head with Republican real estate tycoon Donald Trump in an unprecedented showdown for the White House.

But Sanders was not ready to capitulate, insisting the Democratic nominee will not be chosen until delegates vote at the party's national convention in late July.

And while her campaign acknowledged as "an important milestone" the U.S. network tallies that pushed her beyond the magic number, Clinton said the Democratic race was not yet over. "We are on the brink of a historic, historic, unprecedented moment," she told a rally in Long Beach, California. "But we still have work to do, don't we?" she said.

The capital Washington rounds out the nominating contests when it votes on June 14. Clinton mounted a hectic campaign push in California, keen to finish strong and end any argument for Sanders to remain in the race, as he has pledged to do until the Democratic convention.

"It's not over until it's over," Clinton told reporters at a community center in Compton, near Los Angeles, as she pleaded for supporters to vote Tuesday.

Nancy Worley, chair of Alabama's Democratic Party, is one of the so-called super-delegates - current and former elected officials and political activists who are not bound to vote for a specific candidate - who in a last-minute flurry pushed Clinton over the threshold.

She explained how she had yet to commit to a candidate until Monday, when she received phone calls from three U.S. news outlets.

"If the popular vote is overwhelming and the delegates are very much in her camp, in my opinion, it's kind of crazy not to unify the party and move forward to defeat Donald Trump," Worley told AFP, noting how Democrats in her state chose Clinton by a wide margin.

Clinton edged to the brink of the nomination Sunday when she won the U.S. territory of Puerto Rico. She reportedly surpassed the threshold Monday after a number of super-delegates committed to back her candidacy.

The Sanders campaign called it "a rush to judgment."

Clinton will be dependent on super-delegates "who do not vote until July 25 and who can change their minds between now and then," Sanders spokesman Michael Briggs said in a statement.

Clinton said Monday that she has earned 3 million more votes than Sanders and is well ahead in the pledged delegate count. But Sanders has long argued the system is tilted against him, with hundreds of super-delegates aligning with Clinton before he even entered the race last year.

Trump became the presumptive Republican nominee last month. But the provocative billionaire has stirred controversy since then, including belligerent attacks on a judge presiding over a case against the Trump University real estate program. Trump has claimed the judge, Gonzalo Curiel, is a "Mexican" who is biased against him because of Trump's call to build a wall on the border with Mexico.

Trump's position triggered stinging criticism from fellow Republicans accusing him of racism, highlighting the potential challenge in unifying Republicans behind such a polarizing figure in the general election.

Clinton joined in the Trump-bashing, telling supporters at a rally in southern Los Angeles that "we need to stop this divisiveness, this bullying and bigotry."