Americans are voting in primaries that play a major role in who will challenge US President Donald Trump for the presidency, a day after key endorsements dramatically boosted Joe Biden's hopes against surging leftist Bernie Sanders.
The Democratic establishment is desperate to unite around a moderate candidate who can triumph over front-runner Sanders, a self-described democratic socialist, and face President Trump in November.
Voting began as early as 6am (11am Irish time) in some areas.
As the five remaining Democratic candidates made their final pitch to voters in 14 states, Mr 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 Mr Sanders, who could take a potentially insurmountable lead in the all-important delegate count after Super Tuesday.
Mr Biden has been riding high with key endorsements that built into a political crescendo yesterday.
He took the stage at a rally in Dallas, Texas joined by Minnesota Senator Amy Klobuchar, who told the crowd she was ending her campaign and endorsing Mr Biden for president.
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Mr Biden later introduced Beto O'Rourke, who made a big splash early in the Democratic race but then fizzled out.
Mr O'Rourke remains popular in Texas, the state with the largest delegate haul today after California.
"We need somebody who can beat Donald Trump (and) in Joe Biden we have that man," Mr O'Rourke said, peppering his short speech with Spanish.
Hours earlier Pete Buttigieg, the 38-year-old former mayor of South Bend, Indiana, threw his support behind Mr 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," Mr Buttigieg said in Dallas, with the former vice president at his side.
Mr Biden appeared reinvigorated by the support. But he delivered a tough message intended to warn voters away from the seductions of Mr Sanders, who has called for nothing less than a "political revolution" in America.
"Most Americans don't want the promise of a revolution," Mr 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 Mr Biden.
His campaign was on life support after disappointing showings in the first three state contests, but he is suddenly the main challenger to Mr Sanders on the biggest day of the primary campaign.
Mr Buttigieg had strong showings in predominantly white early states but was unable to mobilise black and Hispanic support.
Ms Klobuchar's campaign never gained traction. By endorsing Mr Biden, she could deprive Mr Sanders of a large delegate claim in her home state of Minnesota today.
New York billionaire Mike Bloomberg, who today competes in his first primaries, has also spread his message to voters in a lavish multi-state ad blitz.
Super Tuesday: The View from Virginia
Virginia is one of the biggest states voting on Super Tuesday, with 99 delegates.
In an Arlington high school cafeteria, which has been converted into a polling station, there's a piano in the corner of the room.
At a quiet moment during a long day, an election worker starts playing.
It’s the theme tune from the Charlie Brown cartoon series, a light-hearted soundtrack for the voters who have gathered to cast their ballots in the race for the Democratic nomination.
Despite the child-like background music, the Democrats here have a serious mission: to get Donald Trump out of office in November.
Jackie Johnson tells me that she was an Elizabeth Warren supporter but has switched her allegiances in recent days because she wanted to back a candidate that could win the nomination and the White House.
"I voted for Mike Bloomberg. These are challenging times and a challenging approach is what we need," she said.
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Martina Smith also voted for Michael Bloomberg.
"I'm not sure he’s the right person but I felt he’s the strongest to stand up to Trump and I’ll vote for a rabid dog before I’ll vote for Donald Trump," she said.
Joe Biden is hoping that endorsements from three former candidates will give him a boost.
Virginia couple John Kettering and Sharon Perales were backing Amy Klobuchar until she dropped out yesterday.
"I voted for Joe Biden, after my candidate endorsed him last night. It was a last-minute change for me," Sharon said.
Derrick Malis voted for Bernie Sanders.
"He’s a socialist and so am I. I think he’ll bring transformation to this country," he said.
'Massive effort' to stop Sanders
Flush with money for ads, an extensive organisation, and momentum in the polls, Mr Sanders has focused on multiple states including delegate-rich California, today's biggest prize.
In Utah, he said there was "a massive effort" trying to stop his campaign.
"The corporate establishment is coming together, the political establishment is coming together," he added. "They are really getting nervous."
Mr Sanders leads Mr Biden nationally in polling.
Mr Biden hopes his newfound momentum can carry him through Super Tuesday with a delegate count close to Mr Sanders, if not the outright lead.
The former vice president to Barack Obama says his strength with blacks, Hispanics, women and suburbanites will show in the coming contests.
Though Ms Klobuchar joined Mr Biden's camp, Mr Sanders has launched an appeal to to her voters.
"I hope her supporters will join us in our fight to defeat Donald Trump in November and win real change," Mr Sanders tweeted, before holding a rally on Ms Klobuchar's home turf.
Third-placed Bloomberg is competing against Mr Biden for moderate voters and campaigned in Virginia on the eve of Super Tuesday.
"I've won three elections so far. I don't plan to start losing now," the former New York mayor said.
Mr Bloomberg has spent an unprecedented $500 million (€449 million) of his own fortune saturating the airwaves with TV adverts.
