WASHINGTON (REUTERS, AFP) - Hillary Clinton has reached the number of delegates needed to clinch the US Democratic presidential nomination, the Associated Press said on Monday (June 6), putting her on course to become the first woman to head a major US party ticket.

But the campaign of her rival, Bernie Sanders, vowed to keep up the fight, saying it was wrong to count the votes of superdelegates before they actually cast ballots at the Democratic National Convention in July.

"Our job from now until the convention is to convince those superdelegates that Bernie is by far the strongest candidate against Donald Trump," the campaign said in a statement. Superdelegates largely consist of party leaders and elected senators, members of Congress and governors.

Clinton, a former secretary of state, reached the 2,383 delegates needed to become the presumptive Democratic nominee with a decisive weekend victory in Puerto Rico and a burst of last-minute support from superdelegates, the AP reported.

Clinton has 1,812 pledged delegates won in primaries and caucuses. She also has the support of 571 superdelegates, according to the AP count.

She would be the first woman nominated for president by a major US political party.

"According to the news, we are on the brink of a historic, historic, unprecedented moment," Clinton told a rally in Long Beach, California, shortly after the AP report. "But we still have work to do, don't we? We have six elections tomorrow and we're going to fight hard for every single vote, especially right here in California."

Clinton mounted a hectic campaign push in California, keen to finish strong and end any argument for Sanders to remain in the race. In addition to voters in California, Montana, New Jersey, New Mexico, North Dakota and South Dakota are all in play Tuesday.

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

The capital Washington rounds out the nominating contests when it votes on June 14.

The Democratic Party holds its convention in Philadelphia in July to formally choose its nominee for the Nov 8 election against presumptive Republican nominee Donald Trump.

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 US 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 US territory of Puerto Rico.

Sanders, looking for big victories Tuesday, contends he will use the coming weeks to try and flip many of Clinton's super-delegates in his favour.

Clinton has been a polarising figure over her three decades in the public eye, and lingering scandals include her use of a private email account while serving as secretary of state.

Questions about her transparency and honesty have pushed up her unfavourability numbers, which rival Trump's.

On Monday she vowed to "do everything I can to unify the Democratic Party," saying she would reach out to Sanders.

"We have to be unified going into and out of the convention to take on Donald Trump and to repudiate the kind of campaign he's running."

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.

Curiel was born in the US state of Indiana to Mexican parents.

Trump's position triggered stinging criticism from fellow Republicans accusing him of racism, highlighting the potential challenge in unifying Republicans behind such a polarising 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." At a family-owned burger joint in Watts, a man praised Clinton for her recent denunciation of Trump as "temperamentally unfit" to be commander in chief.

"I'm gonna go after him all the time," she told the man. "All the time."