Ten Democratic candidates will battle within minutes in the first TV debate of the race to oust President Donald Trump from the White House next year.
Elizabeth Warren, a liberal firebrand who has been gaining momentum, will take centre stage at the primetime forum in Miami, Florida.
The debate will offer much-needed political oxygen to underdogs such as Cory Booker and Beto O’Rourke.
Ten other Democrats in the crowded field will face off on Thursday night.
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Millions of voters are expected to tune in for America’s biggest political debates in years. The first begins at 21:00 (01:00 GMT) on Wednesday and lasts two hours.
Who’s first up?
Ms Warren, a Massachusetts senator, will be looking to cement her surge in opinion polls after unveiling a plethora of policy proposals such as free universal healthcare, higher taxes on the rich and breaking up tech giants.
“This is just a chance,” she said on Tuesday in Miami, “to be able to talk to people all across this country about how this government works better and better and better for a thinner and thinner slice at the top, and it’s just not working for the rest of America.”
The other nine candidates on stage will be hoping to grab some crucial limelight, possibly by taking aim at pack leaders such as Ms Warren.
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Mr Booker won airtime last week when he pilloried Democratic front-runner Joe Biden for touting his work with racist senators decades ago.
But the New Jersey senator has so far failed to break into the top tier.
Mr O’Rourke became a liberal golden boy during an unsuccessful Senate bid last year, but the former congressman’s White House hopes have so far fizzled.
On stage, too, will be Minnesota Senator Amy Klobuchar; former Obama housing secretary Julian Castro, the only Latino in the race; New York Mayor Bill de Blasio; Representatives Tulsi Gabbard and Tim Ryan; Washington state Governor Jay Inslee; and former congressman John Delaney.
They are all at one per cent or lower in opinion polls.
All the top 20 candidates need to demonstrate they have the electoral viability and political grit to defeat the Republican president.
Where will the president be?
Mr Trump, who is running for a second year in office, has said he will watch the debate aboard Air Force One as he flies to Japan for a G20 summit.
“Yes, everyone said I’ll be tweeting,” he told Fox Business Network.
He added: “Do I want to watch it? Do I want to watch these people? That’s a very unexciting group of people.”
He also jabbed at his leading challenger.
“Biden is a lost soul,” Mr Trump said. “He doesn’t know where he is.”
Who will take on Trump?
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Who debates on second night?
Two big beasts of the Democratic political jungle, Joe Biden and Bernie Sanders, will offer competing visions for America on Thursday night.
Mr Biden, a former US vice-president making his third White House bid, is a pragmatic centrist who has been recovering from gaffes and about-turns.
Snapping at his heels is Mr Sanders, a Democratic socialist who is promising a “political revolution” of economic populism, including free healthcare for all and paying off America’s $1.6tn student debt mountain.
Youthful mayor Pete Buttigieg, whose dark horse campaign has been rocked by the police shooting of a black man in his home city of South Bend, Indiana, will also be closely watched.
Senator Kamala Harris, the only black woman in the race; Senator Kirsten Gillibrand, who has campaigned on women’s issues; and entrepreneur Andrew Yang will also be jockeying for attention.
The line-up for both nights was chosen at random by debate host NBC.
The challenge for contenders will be to shine in a field that is largely aligned in opposition to Mr Trump on hot-button issues such as immigration and climate change.
But there are divisions among them on other thorny topics like whether the president should be impeached, or how to tackle America’s healthcare system.
The eventual Democratic White House nominee will be crowned at the party convention in July next year, before the presidential election in November.