Democratic White House front-runners Joe Biden and Bernie Sanders are offering starkly diverging visions for America in a TV debate.
Mr Biden, a moderate, and Mr Sanders, an ultraliberal, are battling for the soul of the party as it ponders how best to take on President Donald Trump.
Eight other candidates are vying on the stage in Miami, Florida, to impress voters ahead of next year’s election.
Sparks flew in the first debate with 10 other Democrats on Wednesday night.
The crowd of contenders will be winnowed until a winner is picked at the party convention in July next year.
He or she will face the Republican president in the November 2020 election.
What’s at stake in Biden v Sanders?
Mr Biden, who served two terms as Barack Obama’s vice-president, is aiming to consolidate his status at the tip of the field on Thursday night after recent missteps.
The 76-year-old has flip-flopped on abortion, recanted after provoking liberal ire for calling Vice-President Mike Pence “a decent guy”, and was strafed for touting his work decades ago with senators who favoured racial segregation.
Mr Biden is an ideological centrist notable for his professed willingness to work with Republicans in order to govern.
He has warned that embracing a more left-leaning platform will alienate the very working-class voters Democrats need to overcome Mr Trump.
As pack leader, the former Delaware senator found a bullseye on his back as he took to the podium before a national TV audience of millions.
He is flanked by nine competitors, including Mr Sanders standing right next to him, and they may sorely test his above-the-fray posture.
On the stump, avowed Democratic socialist Mr Sanders has recently been attacking Mr Biden’s record on trade and his vote for the Iraq war.
The 77-year-old Vermont senator, who lost out to Hillary Clinton in the 2016 race for the party nomination, is promising a political revolution including free healthcare and cancelling student debt.
But Mr Sanders has been losing ground in recent opinion polls to a rival liberal firebrand, Senator Elizabeth Warren.
As the debate began, Mr Sanders confirmed he would raise taxes on American working families, but said they would end up paying less overall because of savings on healthcare.
Biden pledged instead to eliminate Mr Trump’s tax cuts for the wealthy.
Mr Sanders pilloried the president as “a pathological liar and a racist”.
Who will take on Trump?
Who else is on stage?
While the duel between the old guard heavyweights unfolds, younger candidates have a chance to present themselves as the future of the Democratic party.
Two other top-tier candidates are setting out their policy stalls: Pete Buttigieg, the 37-year-old gay mayor of South Bend, Indiana, and 54-year-old Senator Kamala Harris of California.
Both enjoyed brief initial surges in popularity, but their campaigns have so far not lived up to supporters’ high hopes.
Mr Buttigieg, a Harvard-educated military veteran, has been criticised back home for his handling of the police shooting of a black man, exposing fissures in his appeal to African Americans.
Both have been jabbing at Mr Biden. Ms Harris has criticised the former vice-president for supporting a 1994 crime bill now blamed for mass incarceration of black men.
Debate-watchers are keen to see whether the former San Francisco prosecutor will repeat that line of attack to his face.
The only black woman in the field, she has been hotly tipped as a running mate for Mr Biden if he wins the nomination.
The forum will include six other candidates who are all polling at one per cent or less.
They are Senators Kirsten Gillibrand and Michael Bennet, former Colorado Governor John Hickenlooper, congressman Eric Swalwell, self-help guru Marianne Williamson and entrepreneur Andrew Yang.
How did age become an issue?
Mr Biden found himself under early attack on an issue that he has presented as one of his strengths – political longevity.
Mr Swalwell said: “I was six years old when a presidential candidate came to the California Democratic convention and said, ‘It’s time to pass the torch to a new generation of Americans.’
“That candidate was then-Senator Joe Biden. Joe Biden was right when he said it was time to pass the torch to a new generation Americans 32 years ago – he’s still right today.”
Mr Biden retorted: “I’m still holding on to that torch.”
Fellow septuagenarian Mr Sanders interjected: “The issue, if I may say, is not generational. The issue is who has the guts to take on Wall Street, to take on the fossil fuel industry, to take on the big money interests.”
As the debated descended into a free-for-all shouting match, Ms Harris was cheered for saying: “America does not want to witness a food fight – they want to know how we’re going to put food on their table!”
What were the main issues?
Healthcare dominated the first 30 minutes of the debate.
All of the candidates raised their hands when asked who would provide healthcare coverage for undocumented immigrants.
But they argued over whether there should be government-run healthcare, or incremental change to the current US medical system dominated by the private marketplace.
Mr Sanders outlined his plan for Medicare for All, a programme along the lines of Britain’s NHS, though he was short on specifics of how it would be delivered.
But Mr Biden said he opposed any Democrat that was against Obamacare, a veiled jab at Mr Sanders.
Mr Biden pledged to build on Obamacare, saying: “Urgency matters.”
How did the first debate go?
Ms Warren is widely judged to have cemented her top-tier status after emerging from the debating ring on Wednesday night without a glove being laid upon her.
The Massachusetts senator, who has pledged to institute a wealth tax and break up tech giants, railed on stage against nationwide income disparities as “corruption, pure and simple”.
Several lesser-known contenders turned their fire on one another as they vied to emerge from the shadows with only a limited time to make an impression.
Ex-congressman Beto O’Rourke, Senator Cory Booker, former San Antonio mayor Julian Castro and New York Mayor Bill de Blasio clashed on everything from healthcare to foreign policy and immigration.
They were united in opposition to Mr Trump, but disagreed on the extent to which the next president should shift the nation on a more liberal course – a theme sure to resurface on Thursday night.
The first debate hauled in a surprisingly large 15.3 million viewers, according to estimates released by host NBC.
The climactic round two is expected to draw an even bigger audience.
Read more about the other candidates
Democratic debate: Heavyweights Biden and Sanders duel