Ten Democratic candidates are facing off 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, is taking centre stage at the primetime forum in Miami, Florida.
The debate offers much-needed political oxygen to underdogs such as Cory Booker and Beto O’Rourke.
Ten other Democrats in the crowded field will battle on Thursday night.
Millions of voters are expected to tune in for America’s biggest political debates in years. The first began at 21:00 (01:00 GMT) on Wednesday and lasts two hours.
Who’s first up?
Ms Warren, a Massachusetts senator, is 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.
“Who is this economy really working for?” she said in response to the first question. “It’s doing great for a thinner and thinner slice at the top.”
She attacked drug and oil giants and prison contractors before concluding to a big cheer from the studio audience: “We need to make structural change in our government, in our economy and in our country.”
The other nine candidates on stage have been vying to grab some crucial limelight.
Mr O’Rourke showed off his fluency in Spanish, saying that a stable American democracy should “listen to all voices”.
The former congressman became a liberal golden boy during an unsuccessful Senate bid last year, but his White House hopes have so far fizzled.
Mr Booker also focused on inequality, saying the economy was not working for the “low-income black and brown community” where he lives.
The New Jersey senator has so far failed to break into the top tier.
On stage, too, are 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.
Asked about a photo of a migrant father and daughter who drowned trying to reach the US, Mr Castro was cheered when he said their deaths were heartbreaking and “it should also piss us all off”.
All the top 20 candidates need to demonstrate they have the electoral viability and political grit to defeat the Republican president.
What was the president’s reaction?
Mr Trump, who is running for a second year in office, seemed unimpressed as he watched the debate aboard Air Force One en route to Japan for a G20 summit.
He earlier told Fox Business Network: “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?
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.
Front-runner 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.
Read more about the other candidates
US Democrats battle in high stakes TV debate