WASHINGTON (REUTERS) - US Democratic presidential candidate Joe Biden on Tuesday (March 3) took his resurgent campaign to California in a last-minute push to blunt front-runner Bernie Sanders' momentum as Americans voted in the largest round of state nominating contests.

California, the most populous state, is a tantalising prize in the Super Tuesday elections in 14 states that are the first national test for candidates seeking the Democratic nomination to face Republican President Donald Trump in the Nov 3 election.

Biden, who is aiming to muscle aside upstart Michael Bloomberg and consolidate support from moderates, has been re-energized since a blowout win in South Carolina on Saturday, and polls show him gaining on Sanders in some states.

While campaigning in a diner in Oakland, Calif., Biden told a voter that "hopes are high" of him meeting the 15 per cent threshold needed to collect delegates in liberal California. Failure to do so could cement Sanders' lead in the race.

Sanders, a democratic socialist and US senator who has vowed to make the wealthy and corporations assume a heavier tax burden, is hoping progressives, Latinos and young voters put enough wind in his sails to make his second bid for the Democratic nomination successful.

He has heavily outspent Biden on ads and in building a campaign organisation in the Golden State, where 415 delegates will be awarded. At least 1,991 delegates are needed to become the nominee at the party's convention in July.

The rush of primary elections on Tuesday, in which one-third of the delegates are up for grabs, may provide some clarity in a muddled race that has seen several candidates rise and fall, leaving many Democratic voters torn and uncertain.

But Biden, who was President Barack Obama's vice-president, has emerged as a top threat since his South Carolina win opened the floodgates on endorsements from Democratic officials worried that Sanders' proposals to restructure the economy would doom the party's prospects in November.

Biden is trying to thread the needle between progressive Democrats' desire for big structural change in the nation and moderates' yearning for a candidate who will be able to win over enough independents and Republicans to oust Trump.

That effort gained fresh momentum on the eve of Tuesday's voting as moderate presidential rivals Pete Buttigieg, the former mayor of South Bend, Indiana, and Senator Amy Klobuchar of Minnesota, endorsed Biden after withdrawing from the race.

Leslie Cohen, a retired teacher in Sacramento, California, said she had planned to support Buttigieg but would now vote for Biden.

"Once he dropped out and Amy Klobuchar dropped out, my decision was made because I don't want Bernie Sanders. I don't think he can beat Trump," Cohen said.

SIPHONING VOTES

Biden's goal on Tuesday will be to stay within reach of Sanders in the delegate count, giving him a chance to make up ground as the campaign possibly becomes a two-candidate race.

Tennessee is one of the states where Biden hopes to do well on Tuesday. A powerful, killer tornado in the Nashville area delayed the opening of polls there by an hour, and forced officials to relocate some polling locations.

Bloomberg, the billionaire former New York mayor, remains a wild card as he joins the competition for the first time. The moderate skipped the first four contests and spent more than US$500 million (S$700 million) of his own money to bombard Super Tuesday and later voting states with ads, but has seen his poll numbers slip after a poor first debate.

Asked by a reporter in Miami if he thought he risked spoiling Biden's chances of winning the nomination, Bloomberg responded: "You think I'm going to siphon (votes) from him? He's siphoning them from me."

Jeff Sunderland, 39, of Arlington, Virginia, said he voted for Sanders because he believes more needs to be done to improve the plight of workers. "I think that the working people of this country deserve better from our government," he said.

Senator Elizabeth Warren, who was briefly the front-runner in the race last year, also remains in the hunt and hopes to score a victory in her home state of Massachusetts. Opinion polls show her trailing in other states.

The pace of the Democratic race begins to accelerate after Super Tuesday, with 11 more states voting by the end of March.

By then, nearly two-thirds of the delegates will have been allotted.

Voting on Tuesday was taking place against the backdrop of an escalating political and economic crisis over the global outbreak of the coronavirus, which has infected some 90,000 people worldwide and killed more than 3,000, mostly in China.

The Federal Reserve cut interest rates by half a percentage point on Tuesday in an emergency move to shield the world's largest economy from the fallout.

SANDERS LEAD

Sanders headed into Tuesday with 60 delegates to Biden's 54 in the state-by-state nominating fight. Sanders managed a virtual tie with Buttigieg in Iowa and wins in New Hampshire and Nevada.

Besides leading in polls in California, Sanders also is ahead of Biden by a smaller margin in polls in Texas, the second-biggest prize. Sanders' strength with Hispanics should pay dividends in that state, where Latinos comprise one-third of the Democratic electorate.

Biden, whose South Carolina win affirmed his popularity with black voters, hopes to win five states where African Americans make up at least a quarter of the Democratic electorate: Alabama, North Carolina, Virginia, Tennessee and Arkansas.

Other states voting on Tuesday are Colorado, Maine, Minnesota, Oklahoma and Utah. The US territory of American Samoa was holding a caucus contest, and Democrats living abroad began voting in a primary set to continue until March 10.

The first polls will close in Vermont and Virginia at 7pm EST (8am on Wednesday, Singapore time). The last will close in California at 8pm PST (midday on Wednesday, Singapore time).

The next contests on March 10 will be in Idaho, Michigan, Mississippi, Missouri, North Dakota and Washington state.