WASHINGTON (AFP) - For Democrats seeking to challenge Mr Donald Trump in 2020, the rubber meets the road in Miami this week, where Mr Joe Biden will defend his front-runner status as presidential candidates finally square off face to face.

Americans are bracing for the nation's biggest political debate since the slug-fests of 2016, a two-night showdown beginning on Wednesday (June 26), with 20 Democrats vying for a breakout moment that could showcase their talents, or see them stumble on the world stage.

Former vice-president Biden, now in his third White House bid, is the candidate to beat in the most crowded, most diverse field in modern US election history.

The 76-year-old leads comfortably in all polls, and will be under pressure to convince millions of voters watching in prime time that he deserves to remain in pole position despite an uproar over his recent comments about the "civility" between him and avowed segregationists in the US Senate.

Like most of his rivals, Mr Biden spent much of last weekend sharpening his message ahead of the debate by wooing residents of early-voting South Carolina.

"I'm here to tell you, I hope to be your nominee," Mr Biden told applauding voters in Columbia.

"But here's the deal: whomever the Democratic nominee is, we have to stay together and elect a Democrat president of the United States."

Nipping at Mr Biden's heels is a pack of candidates including liberal Senator Bernie Sanders, who is polling second; progressive rising star Senator Elizabeth Warren; young gay mayor Pete Buttigieg; and others desperate to emerge as a viable alternative to the man who was Mr Barack Obama's vice-president.

With 24 Democratic contenders now in the race, the debate will have a freewheeling feel. Party organisers chose the top 20 based on poll numbers and fundraising, splitting them into two groups of 10.

Ms Warren, polling third, will take centre stage on the first night at 9pm, inheriting a prime spot to make her case as she debates ex-congressman Beto O'Rourke, senators Cory Booker and Ms Amy Klobuchar, New York Mayor Bill de Blasio and former San Antonio mayor Julian Castro, each of whom is trying to breathe new life into their campaigns.

But the main event is likely to be Day 2, when the more moderate Mr Biden goes head to head with Mr Sanders, 77, and Mr Buttigieg, an up-and-coming mayor of South Bend, Indiana, who is half Mr Biden's age.

On issues like healthcare and economic inequality, the Democratic socialist Sanders pushed the party towards more progressive positions following his protracted 2016 nomination battle with Mrs Hillary Clinton.

He has pledged to use the debate as a platform to emphasise his support for universal healthcare, his opposition to the Iraq war and to "disastrous trade agreements". In short, he will emphasise his differences with rival Biden.

"For Biden, the debates are a good test of how secure his support is," Mr Kyle Kondik, of the University of Virginia's Centre for Politics, told AFP, adding that he believes Mr Biden has a "tenuous" early lead in the nomination battle.

Tensions have simmered recently after Mr Biden's controversial remarks about the "civility" he shared with two segregationist senators decades ago, when he first joined the Senate.

The comments earned rebukes from rivals Cory Booker and Kamala Harris, the two prominent African Americans in the race.

They and the other younger candidates - Mr Buttigieg, Mr O'Rourke, Mr Castro, Senator Kirsten Gillibrand and congresswoman Tulsi Gabbard - will be looking to draw contrasts with their elders at the debate broadcast on NBC, MSNBC and Telemundo.

'DEFEAT DONALD TRUMP'

Like Mr Biden, Mr Buttigieg enters the event singed with race-related controversy, after the fatal shooting of a black man by a white officer in South Bend led to black residents jeering the mayor at a town hall meeting.

It is the first national-scale test for Mr Buttigieg, a military veteran who rose from being a virtual unknown earlier this year to a top-tier contender.

Some lesser known non-politician candidates could spark interest, including entrepreneur and automation soothsayer Andrew Yang.

Tension has simmered on the party's left flank, where Mr Sanders and Ms Warren are the two flag bearers. Each has called for stronger controls on financial markets, expanded healthcare, and partial or total elimination of student loan debt.

"Whether it's Biden or Elizabeth Warren or anybody else, what I believe is that in fact I am the strongest candidate to defeat Donald Trump," Mr Sanders told CNN last week.

The President himself will not be involved in the biggest political showdown yet in 2019, although he hinted he'll be watching and could weigh in via Twitter.

"I wasn't thinking about it, but maybe I will now," Mr Trump teased to Fox News recently.