Democratic White House front-runners Joe Biden and Bernie Sanders are expected to offer starkly diverging visions for America in a TV debate.
Mr Biden, a moderate, and Mr Sanders, an ultraliberal, will battle for the soul of the party as it ponders how best to take on President Donald Trump.
Eight other candidates will vie 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, will aim 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 may find a big bullseye on his back as he takes to the podium before a national TV audience of millions.
He will be 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.
She was first choice in a recent straw poll by the influential progressive group MoveOn, with Mr Sanders coming in second.
Who will take on Trump?
Who else will be on stage?
While the duel between the old guard heavyweights unfolds, younger candidates will have a chance to present themselves as the future of the Democratic party.
Two other top-tier candidates will set 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 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 second bout is expected to draw an even bigger audience.