WASHINGTON (BLOOMBERG) - United States presidential hopeful Joe Biden scored two quick and significant victories in the first results from Super Tuesday, winning the Democratic primaries in Virginia and North Carolina. Rival Bernie Sanders won his home state of Vermont.

Television networks declared Mr Biden the winner shortly after polls closed in the two states, an indication that his victories were decisive and could net him a large portion of the total 209 delegates available in both states. But states with polls closing later, such as Texas and California, could aid Mr Sanders.

"We're feeling optimistic," Mr Biden said in Los Angeles after the Virginia races were called. "I think we're going to do well in some other states as well."

Tuesday's (March 3) contests across 14 states, plus American Samoa, will award more than a third of all delegates to the Democratic convention in July in Milwaukee. The biggest day of the presidential primary calendar will define the nomination fight for Mr Sanders and Mr Biden and determine whether Mr Michael Bloomberg and Ms Elizabeth Warren have a rationale for carrying on their campaigns.

Mr Sanders and Mr Biden headed into the night close in delegate tallies. Mr Sanders emerged from the first four contests with 60 delegates, but Mr Biden's big win in South Carolina last Saturday brought him within six delegates of his chief rival.

That victory also brought Mr Biden a clearer field and the endorsements of Ms Amy Klobuchar and Mr Pete Buttigieg, two rivals who were competing with him for the votes of centrist Democrats.

That still might not be enough. Mr Sanders holds the advantage in key contests - including delegate-rich states like Texas and California - and Mr Bloomberg threatens to play spoiler. Mr Biden is counting on his support from African-American voters, who make up a sizable portion of the Democratic electorate in Alabama, North Carolina, Tennessee, Virginia and Arkansas.

Mr Biden won 63 per cent of the black vote in Virginia, according to network exit polls published by CNN. A majority of Virginia Democrats said they would prefer a nominee who can beat President Donald Trump in November over a candidate who agrees with them on the issues.

Delivery of results may be affected by vote-counting and weather. The outcome in California, the biggest prize of the night, may not be known for hours after polls close or even days as officials wait for mail-in ballots to arrive and be counted. In Tennessee, polling places in some parts of Nashville will stay open until 10pm local time after tornadoes that swept through the area caused extensive damage and killed at least 22 people.

The Super Tuesday round of primaries marks the first time Mr Bloomberg has appeared on the Democratic presidential ballot. He and Ms Warren, whose best performance to date was third in the Iowa caucuses, will confront the question of whether to continue in the race if they don't collect a sizable number of delegates.

CONVENTION BATTLE

Both have vowed to press on. Mr Bloomberg acknowledged on Tuesday that the only way he can secure the Democratic nomination is if neither Mr Biden nor Mr Sanders wins a majority of delegates and the decision is left to a battle at the party convention in July.

"I don't think that I can win any other ways, but a contested convention is a democratic process," Mr Bloomberg said during a stop at a campaign office in Miami. "There are rules of the Democratic Party of how you go about this."

Mr Bloomberg is the founder and majority owner of Bloomberg LP, the parent company of Bloomberg News.

Beyond Super Tuesday, there are several major caches of delegates available later this month with Michigan, Florida, Illinois and Ohio among the states holding primaries.

"After Super Tuesday, they're all opportunities. I think I'll do well in Florida, Michigan, it goes down the line," Mr Biden said on Tuesday.

Mr Sanders - a self-described democratic socialist who many in the party fear would return Mr Trump to the White House - seems certain to emerge on Tuesday with the most delegates. Mr Biden's task is to win enough support to stand as a credible challenger to Mr Sanders as the primary calendar moves to contests that stretch through June, a task that would be made easier by the growing support he's received just since Saturday night.

The Vermont senator will no longer benefit from a fractured field of moderates, and it will be up to Mr Biden to harness the momentum from his South Carolina victory and turn Ms Klobuchar's and Mr Buttigieg's endorsements into actual votes.

Yet the former vice-president has repeatedly bungled advantages throughout the nominating contest, and the impact of the last-minute withdrawals might be muted with millions of early-voting ballots already cast. In 2016 in California, 60 per cent of voters cast their ballots before Election Day by early or mail-in voting.

As of Monday, Mr Biden was leading in polls in at least two states, North Carolina and Virginia. But polls taken since his South Carolina victory show him closing the gap in other states.

The stakes couldn't be higher for a party still reeling from Mr Trump's election, and weighing diametrically opposite approaches to winning back the blue-collar, working-class voters along the Rust Belt that had long served as its electoral firewall - Mr Biden's centrist vision or Mr Sanders' populist one.

And if voters Tuesday fail to winnow the field, the odds of a contested convention - a political occurrence unseen in nearly 70 years - will soar, further emboldening the incumbent president and diminishing the odds of a successful opposition challenge.

Although he's spent more than US$500 million (S$693 million) on advertising and building a campaign organisation to make his mark on Super Tuesday, Mr Bloomberg said he wasn't sure he would win outright in any state.

"You don't have to win states; you have to win delegates," he said. "I'm in it to win."

He dismissed questions about whether he would peel votes from Mr Biden if he stays in the race, saying that the former vice-president could be seen as taking votes away from him.

"Have you asked Joe whether he's going to drop out?" he said.

Mr Biden told a cheering crowd in Houston on Monday that the country is weary of division, "and hungry, hungry to be united".

In Dallas, he said: "Just a few days ago, press and the pundits declared this campaign dead. But South Carolina had something to say about it. And tomorrow, Texas and Minnesota and the rest of Super Tuesday states, you're going to have a lot to say about it. And when you do, we will be on our way to defeating Donald Trump and a second term."