“In the best-case scenario, Bernie dominates the debate and the arguments. He has one or two good one-liners,” said Mark Longabaugh, a former senior adviser to Sanders’ 2016 campaign. “And obviously the flip side would be Biden has a lackluster debate and maybe makes a mistake. All that coming together maybe causes some pause and hesitation on the part of voters.”

It’s a tall order: Biden has won 14 of the 20 states that have voted this month, and he’s polling about 20 to 40 percentage points ahead of Sanders in the states that cast ballots on Tuesday.

To make matters worse for Sanders, Tuesday was going to be difficult for him even in the best-case scenario: Its biggest delegate prize is Florida, home to many older, moderate Democrats and Cuban exiles who eschew his left-wing politics. It is also a closed primary where only Democrats can vote, and Sanders typically performs better in races where independents can participate.

The reality of the situation isn’t lost on Sanders’ aides. “We’re not naïve,” said Faiz Shakir, Sanders’ campaign manager. “It’s going to be a hard path.”

“We’re not naïve." Bernie Sanders' campaign manager Faiz Shakir

During an all-staff call Thursday evening, Sanders’ national organizing and field director, Misty Rebik, admitted to the team that it was facing an uphill battle — but it has always been that way, she tried to assure them. Sanders himself has appeared less confident on the campaign trail in the last few weeks, even looking downcast at a round-table with workers in Michigan, which is the type of event where he usually seems most upbeat.

At the same time, Sanders’ staffers truly believe the trajectory of the race could change — that the campaign that somehow found itself in a stronger position after Sanders suffered a heart attack last fall could have another improbable comeback.

They argue that the primary has been volatile, and that Biden himself trailed in the first three primary and caucus states before leapfrogging to frontrunner. They also point out that former presidential candidates such as Kamala Harris and Amy Klobuchar have surged in polls after strong performance debate performances.

“I don’t know if you’ve seen Joe Biden lately, but he doesn’t like to be told he’s wrong,” said a Sanders adviser. “We’re going to take him on around the issues and make him defend his record. He’s deflected on those in earlier debates because there’s so much chatter happening on the stage. But when you’re one-on-one, you change the dynamic.”

It’s unclear how aggressive Sanders will be with Biden. Some of Sanders’ staffers and supporters have been frustrated by what they see as his light touch with his former Senate colleague, whom Sanders likes and considers a friend. Even with Hillary Clinton, Sanders often lacked a killer instinct, famously telling her in a 2015 debate that “the American people are sick and tired about hearing about your damn emails!”

Sanders, who doesn’t practice in mock debates while readying for showdowns, did not change his strategy during debate prep last week. “I know it’s shocking for you to hear that Bernie Sanders remained consistent in his approach,” joked Shakir.

Adam Green, co-founder of the Progressive Change Campaign Committee, a group closely aligned with Elizabeth Warren, urged Sanders to stay in the race to debate the former vice president and prevent a “quick Biden coronation.” Given that Sanders this week laid out several questions he plans to ask Biden during the debate, Green doesn’t “expect Bernie to attempt a knockout punch like Elizabeth Warren did to Michael Bloomberg.”

But, he added, “In this new coronavirus world, he has a very legitimate case that Democrats defeat Trump if we stand for universal free health care, paid sick leave, student debt cancellation, and a government that actually works for people. If he presses Biden in good faith to agree, he both makes Biden more electable and proves himself more electable — a win-win that lessens objections to him staying in the race this month."

In the Sanders campaign’s ideal world, aides said, he would win the debate, which would give him enough momentum to carry at least one of the four states voting on Tuesday. His aides see Arizona — partly because he has performed strongly among Latino voters — and Illinois as the best possibilities.

In their view, that could change the narrative about Sanders’ campaign being all but done, and allow them to potentially win states voting later this primary, such as Wisconsin, Pennsylvania and New York.

To gain steam, "you have to do it by initially suggesting the storyline was wrong about Bernie" being finished, said Shakir. “Right now, I’m not sure that anyone — correct me if I’m wrong — is suggesting that we will win in any of those four states. Hopefully we can turn that around.”

Another reason the Sunday debate looms so large in the Sanders team’s mind is because his aides believe that voters who have seen Biden up close in states such as Iowa and New Hampshire have not supported him.

A Biden adviser said of that notion, "South Carolinians have seen Joe Biden up close for decades. They chose him overwhelmingly."

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In a statement, the Biden campaign signaled that the former vice president is not planning on hammering away at contrasts with Sanders and instead wants to focus on Trump. Biden is "honored to share the stage with Sen. Sanders, who he considers a good friend and whose values and leadership he admires," said Andrew Bates, Biden’s rapid response director.

For Sanders, the debate is also an opportunity to nudge Biden to the left, a familiar position for the politician who successfully pulled Clinton in his direction during his unsuccessful 2016 bid. Sanders sees the coronavirus outbreak as a red flag that America’s health care system is dysfunctional, and that the country needs “Medicare for All” and other liberal policies.

“There are 28 million people who are currently uninsured. How are we going to pay for their treatment and their testing? How are we going to deal with seniors who are seeing their retirement accounts hit, other than by expanding Social Security?” asked Rep. Ro Khanna (D-Calif.), a Sanders campaign co-chair. “The coronavirus and the uncertainty around it, the economic uncertainty, gives a way of having Sen. Sanders talk about his policies. Not just in an aspirational, abstract sense, but in a way of here and now.”

During the Thursday all-staff call, Shakir and other senior aides said that campaigning during the coronavirus pandemic meant that they would need to focus more on their digital operation and “distributed organizing” strategy, which, they argued, are superior to Biden’s.

The digital gap between the two campaigns was on display Friday, when the Biden operation struggled with technical difficulties during its first virtual town hall. Since Sanders first announced he was running in 2020, his team has livestreamed almost every public event.

But if the coronavirus has shifted voters toward a candidate so far, it’s Biden. Exit polls found Democratic primary voters trust Biden more than Sanders to lead during a crisis by double-digit margins. Sanders’ team aims to change that — and everything else — on Sunday.

"You don't run for president if you're not in it to win,” said Khanna. "He’s hoping the debate will change the dynamic.”

Laura Barron-Lopez contributed to this report.