Joe Biden’s Democratic presidential bid has picked up steam, with the former US vice-president set to pick up the endorsements of two former 2020 rivals: Amy Klobuchar and Pete Buttigieg.

Ms Klobuchar, a senator from Minnesota, will become the third 2020 candidate in as many days to drop out of the race when she announces the suspension of her campaign in Dallas on Monday night (local time), where she will also publicly back Mr Biden, an aide said.

Mr Buttigieg, a former Indiana mayor who ended his White House bid on Sunday, also plans to endorse Mr Biden in Dallas, a top adviser said.

“The Klobuchar campaign confirms the Senator is flying to Dallas to join Vice-President Biden at his rally tonight, where she will suspend her campaign and endorse the Vice-President,” a campaign aide told Reuters in a text message.

Mr Biden is fresh off a resounding victory in Saturday’s South Carolina primary and aiming for a strong showing on Super Tuesday against Senator Bernie Sanders, the national front-runner and a democratic socialist from Vermont.

The Super Tuesday contests offer the biggest one-day haul of the 1991 delegates needed to win the Democratic nomination at its national convention in July, with about 1357 delegates up for grabs.

Fourteen states – California, Texas, Virginia, Massachusetts, Tennessee, Alabama, Arkansas, Oklahoma, Minnesota, Vermont, Colorado, Utah, North Carolina and Maine – as well as American Samoa and Democrats living abroad cast ballots on Tuesday.

Billionaire former New York Mayor Mike Bloomberg will make his ballot-box debut on Super Tuesday. He has been betting that the hundreds of millions of dollars of his own money he has poured into his campaign will allow him to make up for not competing in the first four early-voting states.

Five candidates – Mr Biden, Mr Bloomberg, Mr Sanders, Senator Elizabeth Warren and Tulsi Gabbard – remain in the running for the nomination to take on Republican President Donald Trump in November’s election. The numbers have already whittled down from more than 20 earlier in the race.

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Mr Bloomberg and Mr Biden have emerged as the main contenders for the votes of moderate Democrats, while Mr Sanders is the progressive front-runner nationally.

Mr Biden’s high-stakes triumph in South Carolina, where his campaign had said his popularity with black voters would propel him to victory after early disappointing finishes in Iowa and New Hampshire, helped see off the competition.

In addition to Ms Klobuchar and Mr Buttigieg, billionaire environmental activist Tom Steyer also gave up his campaign on Saturday after a third-place finish in the Southern state in which he had invested most heavily.

But it was not immediately clear who would immediately benefit from their departures. A Morning Consult poll taken on February 23-27, for example, before Mr Buttigieg exited the race, showed that 21 per cent of his supporters named Mr Sanders as their second choice, 19 per cent picked Mr Biden, another 19 per cent chose Ms Warren and 17 per cent favoured Mr Bloomberg.

Mr Biden still lags his rivals in spending and organisation in Super Tuesday states and beyond. His campaign said on Sunday it had raised more than $10 million in the preceding two days.

-with AAP