LOS ANGELES: Hillary Clinton has secured enough delegates to win the Democratic presidential nomination, according to US media outlets, but her campaign urged supporters to get to the polls on Tuesday to avoid a loss to Bernie Sanders in California as she seeks to unite the party.

Clinton campaign manager Robby Mook said they were pushing supporters and volunteers to “stay at this” as California, New Jersey, Montana, North Dakota, South Dakota and New Mexico hold nominating contests.

A former US secretary of state, Clinton would be the first woman presidential candidate of a major US political party.

“We're on the verge of making history and we're going to celebrate that tonight,” Mook told CNN.

“There are a lot of people we want to make sure turn out today. We do not want to send a message that anybody's vote doesn't count.”

California is the biggest prize on Tuesday — the last and largest state to vote in what became a surprisingly protracted and bitter Democratic primary race to pick a nominee for the November 8 presidential election.

If Sanders, who was trailing in polls in California until recently, won the state, it could hamper Clinton's ability to unify the party ahead of next month's party convention.

Clinton is anxious to turn her full attention to the general election campaign against presumptive Republican nominee Donald Trump.

She secured the endorsement on Tuesday of US Representatives Nancy Pelosi of California, who as House Democratic leader withheld her support until voting day.

President Barack Obama himself was eager to start campaigning, the White House said, but wanted to give voters an opportunity to cast ballots before weighing in on the Democratic race.

Sanders, a US senator from Vermont who describes himself as a democratic socialist, has vowed to continue the fight until the party convention that formally picks the nominee.

He has commanded huge crowds in parks and stadiums, galvanising younger voters with his promises to address economic inequality.

But Clinton has continued to edge out Sanders, particularly among older voters with longer ties to the Democratic Party. She has led a more pragmatic campaign, focused on building on Obama's policies.

Rush to judgment

After the Associated Press and NBC reported on Monday night that Clinton had clinched the number of delegates needed to win the nomination, a Sanders campaign spokesman castigated what he said was the media's “rush to judgment.”

Under Democratic National Committee rules, most delegates to the party's July 25-28 convention are awarded by popular votes in state-by-state elections, and Clinton has a clear lead in those “pledged” delegates.

But the delegate count, where Clinton's support outnumbers Sanders' by more than 10 to 1, also includes “superdelegates” - party leaders and elected senators, members of Congress and governors - who in theory can change their mind at any time.

For that reason, the DNC has echoed the Sanders campaign, saying the superdelegates should not be counted until they actually vote at the Philadelphia convention.

In practice, superdelegates who have announced their intention are unlikely to change their mind.

The AP and NBC reported that Clinton had reached the 2,383 delegates needed to become the presumptive Democratic nominee with a decisive weekend victory in Puerto Rico, a US territory, and a burst of last-minute support from superdelegates.

“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.”

Michael Briggs, Sanders' spokesman, dismissed the AP and NBC tallies. “Our job from now until the convention is to convince those superdelegates that Bernie is by far the strongest candidate against Donald Trump,” he said.

Sanders' supporters have become increasingly resistant to Clinton in recent months, with fewer than half saying they would vote for her if she becomes the party's nominee, according to a Reuters/Ipsos poll in May.

Last month, 41 per cent of Sanders' supporters said they would vote for Clinton if she runs against Trump in the general election. That was down from 50 per cent in April, and 52 percent in March.

Those who have decided not to support Clinton are split on what to do if Sanders quits the race. Some may cross party lines and vote for Trump, but many others appear to be interested in a third-party candidate.

Some 27 per cent of Sanders' supporters said in May that they would vote for neither candidate or another alternative.

The Reuters/Ipsos poll included 2,919 Sanders supporters during the month of May and has a credibility interval, a measure of accuracy, of 2 percentage points.