Joe Biden's White House campaign received a dramatic boost on Monday when three of his ex-rivals — former Texas congressman Beto O'Rourke, Senator Amy Klobuchar and Pete Buttigieg — endorsed him for president on the eve of the crucial Super Tuesday primaries.

The move marked an unprecedented turn in a fractured, often bitter campaign, with the Democratic establishment desperate to coalesce around a moderate candidate who can fight off surging leftist frontrunner Bernie Sanders and face President Donald Trump in November.

As the five remaining Democratic candidates made their final pitch to voters in 14 states, Biden was capitalising on momentum he seized at the weekend with a blowout victory in South Carolina.

The 77-year-old former vice president is consolidating support among moderates eager to blunt the advance of Sanders, who could take a potentially insurmountable lead in the all-important delegate count after Super Tuesday.

Biden has been riding high with key endorsements that built into a political crescendo on Monday.

He took the stage at a rally in Dallas, Texas joined by Senator Amy Klobuchar, the Minnesota pragmatist who told the crowd: "Today I am ending my campaign and endorsing Joe Biden" for president.

"If you feel tired of the noise and the nonsense in our politics, and you are tired of the extremes [...] I think you know you have a home with Joe Biden," she added, to a loud cheer and chants of "Amy! Amy!"

After Biden spoke, he introduced O'Rourke, who made a big splash early on in the Democratic race but then fizzled out. O'Rourke remains popular in Texas, the state with the largest delegate haul on Tuesday after California.

"We need somebody who can beat Donald Trump (and) in Joe Biden we have that man," O'Rourke said.

"He's decent, he's kind, he's caring, he's empathetic," O'Rourke said, peppering his short speech with Spanish.

Just two hours earlier Pete Buttigieg, the 38-year-old former mayor of South Bend, Indiana threw his support behind Biden a day after quitting the race himself.

"I'm looking for a president who will draw out what is best in each of us, and I'm encouraging everybody who was part of my campaign to join me because we have found that leader in vice president, soon-to-be-president, Joe Biden," Buttigieg said in Dallas, with the former vice president at his side.

Biden appeared reinvigorated by the support, and he repeatedly flashed his million-watt smile.

But he delivered a tough message intended to warn voters away from the seductions of Sanders, the national frontrunner who has called for nothing less than a "political revolution" in America.

"Most Americans don't want the promise of a revolution," Biden told supporters. "They want results. They want a revival of decency, honour and character."

The trio of endorsements could be political gold for a resurgent Biden.

His campaign was on life support after disappointing showings in the first three state contests, but he is suddenly the main challenger to Sanders on the biggest day of the primary campaign.

New York billionaire Mike Bloomberg, who on Tuesday competes in his first primaries, has also spread his message to voters in a lavish multi-state ad blitz.

'Massive effort' to stop Sanders

Flush with money for ads, an extensive organisation, and momentum in the polls, Sanders has focused on multiple states including delegate-rich California, Tuesday's biggest prize.

"It is no secret [...] that there is a massive effort trying to stop Bernie Sanders," the frontrunner said in Utah about himself.

"The corporate establishment is coming together, the political establishment is coming together," Sanders added.

"They are really getting nervous."

Sanders, whose ascent as a self-described democratic socialist has disconcerted Democratic grandees, is leading Biden nationally in polling.

Bloomberg is in third place, with progressive Senator Elizabeth Warren close behind and congresswoman Tulsi Gabbard far back in fifth.

Buttigieg had strong showings in predominantly white early states but was unable to draw black and Hispanic support.

Klobuchar's campaign never gained traction. By endorsing Biden, she could deprive Sanders of a large delegate claim in her home state on Tuesday.

Resurrection

With Biden's fortunes resurrected, he hopes his newfound momentum can carry him through Super Tuesday with a delegate count close to Sanders, if not the outright lead.

The former number two to Barack Obama says his strength with blacks, Hispanics, women and suburbanites will show in the coming contests.

Though Klobuchar joined Biden's camp, Sanders weighed in to appeal to her voters.

"I hope her supporters will join us in our fight to defeat Donald Trump in November and win real change," Sanders tweeted, before holding a rally on Klobuchar's home turf.

Also courting moderate and independent voters is Bloomberg, who campaigned in Virginia on Super Tuesday eve.

"I've won three elections so far. I don't plan to start losing now," the former New York mayor said.

Bloomberg has spent an unprecedented $500 million of his own fortune saturating the airwaves with TV spots.