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US Democratic presidential candidate Joe Biden has picked up three endorsements from former rivals on the eve of the most important day in the 2020 White House race so far.

Amy Klobuchar, Pete Buttigieg and Beto O'Rourke all campaigned with the former US vice-president in Dallas, Texas.

On Super Tuesday, voters in 14 states will pick their preferred Democratic candidate for November's election.

The Democratic frontrunner remains the left-wing senator, Bernie Sanders.

Democratic centrists have been calling on party figures to urgently unite to try and stop Vermont Senator Bernie Sanders, a self-described democratic socialist, from becoming the party's nominee to take on President Donald Trump, a Republican.

Mr Sanders, meanwhile, has been steadily picking up endorsements from left-leaning Democrats, activist groups and high-profile celebrities,

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• What are primaries and how do they work?

The other candidates still in the race are Elizabeth Warren, Michael Bloomberg and Tulsi Gabbard. With the exception of Ms Gabbard, a Hawaii congresswoman, all of the Democratic hopefuls are septuagenarians.

What did Biden's new backers say?

Ms Klobuchar, a Minnesota senator, suspended her campaign and endorsed Mr Biden on Monday.

Appearing alongside Mr Biden on Monday night, she said: "If we spend the next four months dividing our party and going at each other, we will spend the next four year watching Donald Trump tear apart our country."

Mr Buttigieg, a former mayor from Indiana who dropped out of the race on Sunday, said at another event with Mr Biden: "I'm looking for a leader, I'm looking for a president, who will draw out what's best in each of us."

Mr Biden, 77, told reporters that Mr Buttigieg, 38, "reminds me of my son Beau", who died in 2015.

Former US congressman Mr O'Rourke, who dropped out of the race last November, also appeared on stage with Mr Biden.

Ms Klobuchar and Mr Buttigieg had both been competing with Mr Biden for the same pool of centrist Democratic voters.

Both stepped aside following Mr Biden's overwhelming victory in South Carolina at the weekend, which came after he struggled in the first three Democratic leadership contests this year.

Earlier on Monday, Mr Biden also picked up endorsements from former Obama National Security Adviser Susan Rice, former Senate leader Harry Reid and several members of Congress.

• Choose your candidate and filter by category
• Five charts that explain the Democratic race

Who else is still in the race?

Five contenders remain out of a Democratic field that once featured more than two dozen.

Among them is New York billionaire Mike Bloomberg, who is also pitching to moderate Democratic voters.

The former mayor of New York City - who will appear on a 2020 ballot for the first time on Tuesday - has invested more than a half billion dollars in his presidential bid.

Mr Bloomberg told a Fox News town hall on Monday that the nominee could ultimately be chosen through "horse-trading" at the Democratic convention in Milwaukee, Wisconsin, in July.

Also still in the race are Massachusetts Senator Elizabeth Warren and US congresswoman Tulsi Gabbard of Hawaii.

How did Sanders react to the Biden endorsements?

Campaigning in Minnesota on Monday night, Mr Sanders invited his rivals' supporters to join his campaign.

"To all of Amy and Pete's millions of supporters, the door is open. Come on in," Mr Sanders said.

• Eighteen things Bernie Sanders believes

His campaign suggested the party's big money donors wanted to stop the Vermont senator.

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"The establishment is nervous, not because we can't beat Trump, but because we will," said Sanders' campaign manager, Faiz Shakir.

Mr Sanders is promising to target US income inequality with a multi-trillion dollar policy blueprint on everything from healthcare to taxation and education.

While centrists are now rallying behind Mr Biden, Mr Sanders has been endorsed by a number of progressive Democrats - including Alexandria Ocasio-Cortez, Ilhan Omar and Rashida Tlaib, known for being part of "the squad" of left-wing congresswomen.

The Sunrise Movement - a coalition of young climate activists in the US - has also endorsed Mr Sanders, as have activist groups People's Action and the Democratic Socialists of America (DSA).

Businessmen Ben Cohen and Jerry Greenfield, co-founders of the ice cream company Ben & Jerry's, have also voiced their support for Mr Sanders.

The socialist candidate has also amassed an extremely long list of celebrity supporters.

Actors including Danny Devito, Susan Sarandon and Jack Nicholson have thrown their weight behind him, as has model Emily Ratajkowski, and pop stars Cardi B, Ariana Grande and Miley Cyrus.

How has Trump responded?

At a campaign rally in North Carolina on Monday night, Mr Trump sought to raise doubts about Biden's mental acuity by highlighting the former vice-president's gaffes.

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He mentioned that Mr Biden had referred to Super Tuesday as "Super Thursday" and asserted in a TV debate that 150 million people had been killed by guns since 2007.

Mr Trump said that if Mr Biden wins the White House, "they're going to put him into a home, and other people are going to be running the country, and they're going to be super-left radical crazies".

What next?

After the first four primary contests, Mr Sanders is in the lead with 60 of the delegates needed to win the Democratic White House nomination.

Mr Biden has 54 and Ms Warren eight.

On Super Tuesday, 1,344 delegates will be up for grabs.

California, Texas, Virginia, Massachusetts, Tennessee, Alabama, Arkansas, Oklahoma, Minnesota, Vermont, Colorado, Utah, North Carolina and Maine - as well as American Samoa, cast ballots on Tuesday.

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