Polls have opened in the US state of New Jersey, on a day of decisive primaries in the race for the White House.
Voting began hours after US delegate tallies showed Hillary Clinton securing the Democratic nomination.
However, the former secretary of state has not yet claimed victory, urging supporters to vote and help push her to a strong finish.
Both the Associated Press and NBC have reported that Ms Clinton 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 additional support from super delegates.
But the campaign of her rival, Bernie Sanders, vowed to keep up the fight in what has been a protracted and increasingly antagonised primary race that has exposed deep rifts between the left wing and the more centrist arm of the Democratic Party.
A Sanders campaign spokesman said it was wrong of the news outlets which made the calls, to count the votes of super delegates before they cast ballots at the Democratic National Convention in July.
"Our job from now until the convention is to convince those super delegates that Bernie is by far the strongest candidate against Donald Trump," Mr Sanders' spokesman Michael Briggs said in a statement, castigating what he called the media's "rush to judgement".
While most delegates are awarded by popular votes in state-by-state elections, super delegates largely consist of party leaders and elected senators, members of Congress and governors, and can change their mind at any time.
For that reason, the Democratic National Committee has echoed the Sanders campaign, saying the super delegates should not be counted until they vote at the convention in Philadelphia.
Blog: 'Is she or isn't she? Democratic delegate maths'
Mr Sanders, a US senator from Vermont who calls himself a democratic socialist, has commanded huge crowds spilling out of parks and stadiums and has been particularly bolstered by younger voters angered by widening economic inequality with his promise of a "political revolution".
But Ms Clinton, who prefers smaller, round-table events, has continued to edge out Mr Sanders, particularly among older voters with longer ties to the Democratic party.
"According to the news, we are on the brink of a historic, historic, unprecedented moment," Ms 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 [Tuesday] and we're going to fight hard for every single vote, especially right here in California."
Ms Clinton has 1,812 pledged delegates won in primaries and caucuses, and Mr Sanders has 1,521. She also has the support of 571 super delegates, according to an AP count, compared to 48 for Mr Sanders.
Montana, North Dakota, South Dakota and New Mexico also hold nominating contests today, but most attention will focus on California, the country's most populous state where another 475 pledged delegates are at stake.
Ms Clinton once held a sizeable lead there over Mr Sanders, but opinion polls in recent days showed them in a dead heat.
A Sanders victory there could embolden his supporters to urge him to wage a fractious convention fight. It could also help Donald Trump who clinched the Republican nomination last month, argue that she is a weak candidate.
