Former Vice President Joe Biden and Senator Bernie Sanders are set to face off on Sunday night in the first one-on-one debate of the Democratic primary.
The contest comes as the nation and much of the world struggle to contain a global pandemic amid mounting economic uncertainty.
Much has changed in the US, and in the campaign, since the Democrats’ last debate less than three weeks ago.
Mr Biden will step on stage as the front runner, a distinction that seemed unlikely when Mr Sanders was winning early contests.
But more moderate Democrats have rapidly consolidated around Mr Biden, buoyed by his strong standing with black voters and motivated by a desire to block Mr Sanders.
The senator is a self-described democratic socialist who many in the party believe would lose to President Donald Trump in November.
The fast-moving coronavirus has largely grounded the candidates for days, leading them to cancel campaign rallies even as big states prepare to vote on Tuesday.
It has also frozen the trajectory of the primary contest, limiting Mr Sanders’ opportunities to regain momentum.
For both candidates, the debate is a moment to display their leadership skills in front of what could be one of the largest audiences of the primary.
They will aim to draw a contrast with Mr Trump’s uneven handling of the coronavirus pandemic.
And the Democrats will argue over who has the right experience, temperament and policies to lead the nation through a crisis.
“Moments like these don’t come around often in campaigns and this is a perfect opportunity to show millions that you have what it takes,” said Robert Gibbs, former White House press secretary and campaign adviser to president Barack Obama.
“They must show voters they are the answer to what is missing right now by being calm, honest, ready to lead and empathetic.”
The coronavirus crisis rapidly upended plans for Sunday’s debate.
First, the Democratic National Committee announced that it would hold the contest without a live audience.
Then the debate was moved from a large venue in Arizona, one of the states holding a primary on Tuesday, to a TV studio in Washington because of concerns about cross-country travel.
CNN, which is hosting the debate, said it was setting the podiums for Mr Biden and Mr Sanders two metres apart, in keeping with guidance from the Centres for Disease Control and Prevention for limiting the spread of the coronavirus.
For Mr Biden, the outbreak of a global pandemic has been a moment to bolster the central argument of his candidacy.
He says his eight years as vice president gave him the experience, and the relationships in Washington and around the world, that are needed in the Oval Office during turbulent times.
Advisers say Mr Biden will also aim in Sunday’s debate to show voters who backed Mr Sanders or other liberal candidates that they have a home in his campaign.
In one overture to liberals, he announced his support for a bankruptcy plan championed by Elizabeth Warren, the Massachusetts senator, who ended her 2020 campaign this month and has yet to endorse anyone.
In a virtual town hall on Friday, Mr Biden said his support for Ms Warren’s proposal, which aims to simplify the bankruptcy process, is “one of the things that I think Bernie and I will agree on”.
After rebounding from his sluggish start in the primary, Mr Biden now holds a solid lead over Mr Sanders in the all-important delegate race, and a strong showing in Tuesday’s primary contests could effectively guarantee his nomination.
Four big states will be up for grabs. They are Illinois, Ohio, Arizona and Florida, a perennial election battleground where Mr Biden appears to have an edge over Mr Sanders.
Mr Sanders and his supporters were caught off guard by how quickly the race has moved away from them.
The Vermont senator retains a loyal following among young voters and liberals, but he has failed to expand his appeal, particularly among black voters.
His calls for a sweeping political and economic revolution have also fallen flat with suburban voters.
Mr Sanders is facing some pressure from within the Democratic Party to step aside and allow Mr Biden to take on Mr Trump.
Several Democratic groups who were waiting to give endorsements until after the primary have consolidated around Mr Biden.
Mr Sanders’ advisers say he is a realist about his current standing and the difficulty of the path ahead.
Yet the senator is pledging to grill Mr Biden in Sunday’s debate on his plans for tackling student debt, his past support of the Iraq War and his backing of multilateral trade agreements.
“I’m going to ask: ‘Joe, what role have you played in trying to make sure that we end this massive level of income and wealth inequality, where three people own more wealth than the bottom half of America?’” he said on Saturday during an online “fireside chat” with supporters.
Yet it is unclear whether the issues he is aiming to highlight will resonate with voters at a time when much of the nation’s focus has shifted to the growing toll of the coronavirus.
Schools and businesses across the country are closed, and many hospitals and clinics are struggling to obtain tests for the virus.
For most people, the new coronavirus causes only mild or moderate symptoms, such as a fever and cough.
For some, especially older adults and people with existing health problems, it can cause more severe illness, including pneumonia.
The vast majority of people recover from the new virus.
The World Health Organisation says people with mild illness recover in about two weeks, while those with more severely affected may take three to six weeks to recover.