While Clinton wants to move beyond the primary battle and turn her attention to Trump, Sanders has vowed to stay in until July's party convention that formally picks the nominee
Democrat Hillary Clinton beat rival Bernie Sanders in New Jersey's presidential nominating contest on Tuesday, Fox News projected, bolstering her lead a day after she captured the number of delegates needed to become her party's US presidential nominee.
Clinton, a former first lady, senator and US secretary of state, would be the first woman to become the presidential candidate of a major US political party.
New Jersey was one of six states holding contests on Tuesday, including the big prize of California, where Clinton is still at risk of an embarrassing loss to Sanders as she heads into a campaign against presumptive Republican nominee Donald Trump in the November 8 election.
Other news outlets, including MSNBC and CNN, said it was still too early to call the winner in New Jersey shortly after the polls closed.
Clinton secured enough delegates to the party's convention to win the nomination before Tuesday's voting, US media outlets reported on Monday night.
But Clinton campaign manager Robby Mook said the campaign was pushing supporters and volunteers to "stay at this" for the contests in New Jersey, Montana, North Dakota, South Dakota and New Mexico and California.
"We're on the verge of making history, and we're going to celebrate that tonight," Mook told CNN. "There's 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."
Clinton, who now must try to unify the party and win over Sanders supporters, will highlight the historic nature of her nomination at an event in Brooklyn on Tuesday night. Her campaign has compiled a video tying her to women's rights movements in American history.
She wants to move beyond the primary battle and turn her attention to Trump. But Sanders, a democratic socialist US senator from Vermont, has vowed to stay in until July's party convention that formally picks the nominee, defying growing pressure from party leaders to exit the race.
If Sanders wins the primary in California, America's most populous state, it would not be enough for him to catch Clinton in the overall delegate count but could fuel his continued presence in the race. Polls in California were due to close at 11 pm ET (0300 GMT on Wednesday).
"We will look forward tonight to marking having reached the threshold of a majority of the pledged delegates,” Clinton spokesman Brian Fallon told CNN, referring to delegates won in the state nominating contests. "And at that point, Bernie Sanders will be out of our race."
Sanders, 74, has commanded huge crowds, galvanizing younger voters with promises to address economic inequality. Clinton, 68, has edged him out, particularly among older voters, with a more pragmatic campaign focussed on building on President Barack Obama's policies.
Steven Acosta, a 47-year-old teacher living in Los Angeles, voted for Clinton on Tuesday, saying that was partly because he believed she stood a better chance of winning in November.
"I like what Bernie Sanders says and I agree with almost everything that he says," Acosta said. "The problem is that I think Republicans would really unify ... even more against him."
'Rush to judgement'
Sanders was determined to stay in the race, even 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 judgement."
Under Democratic National Committee rules, most delegates to the July 25-28 convention in Philadelphia are awarded by popular votes in state-by-state elections, and Clinton has a clear lead in those pledged delegates.
But the delegate count also includes superdelegates, party leaders who can change their minds at any time. Clinton's superdelegate support outnumbers Sanders' by more than 10 to 1.
The Sanders' campaign has said it can still persuade superdelegates to switch to him, although in practice superdelegates who have announced their intentions are unlikely to change their minds.
Sanders would have to get more than 60 per cent of the superdelegates backing Clinton to switch their votes. So far, his campaign manager, Jeff Weaver, acknowledged they had not converted a single delegate.
Trump, 69, became his party's presumptive nominee last month, outlasting 16 Republican challengers.