Hours before his primetime debate with former Vice President Joe Biden, Sen. Bernie Sanders, I-Vt., stressed in an email to supporters that “this is a very big day for our campaign and our movement.”

In the last 15 days, the populist senator from Vermont has watched Biden zoom past him in the race for the presidential nomination. Following a landslide win in South Carolina’s primary and sweeping victories in the coast-to-coast Super Tuesday states as well as last week’s mini Super Tuesday, the former vice president cemented his position as the unrivaled front-runner and has taken a commanding lead in the all-important race for convention delegates.

And in the last week, as the outbreak of the coronavirus has upended life for many Americans, it's in many ways pushed the Democratic nomination race to the side lines.

Now, with public opinion polls showing Sanders trailing Biden significantly in the four large states holding primaries on Tuesday, Sunday night’s primetime showdown is likely the progressive lawmaker’s last opportunity to do something none of the other one-time presidential contenders could do in the Democrats’ race: knock out Biden on the debate stage.

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While Biden stumbled numerous times in the debates – think back to Sen. Kamala Harris’ near-knockout of Biden in the first debate last June over his decades-old stance on busing – he’s yet to be flattened.

On the eve of the debate, Sanders emphasized during a virtual fireside chat that he’s “looking forward to this debate for a number of reasons, not the least of which, it’s a two-person debate. I have a real problem, as I think many Americans do, with debates that turn into food fights. When you have seven, eight people up there or more yelling and screaming and trying to get the soundbite of the night, that the media will pick up on.”

He continued, “I think in a two-hour debate with two people, we can explore some of the real issues facing this country.” Sanders has been making his second straight White House bid.

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While Sanders clearly will do his best to challenge Biden to embrace the progressive policies he’s proposed, the coronavirus is giving some voters a thirst for leadership that could steer the country through the most serious pandemic it’s faced in a century.

Both campaigns have acknowledged that the coronavirus, officially known as COVID-19, will be a major part of the conversation in Sunday night’s debate. While both candidates gave major addresses on Thursday laying out their plans for dealing with the crisis, Biden likely will use the debate to spotlight his nearly half-century of government leadership, first as a senator from Delaware and later as vice president for eight years under President Barack Obama.

A senior Biden campaig official telling reporters a couple of hours before the debate that "in a moment of national crisis, it is more clear than ever to Americans that we are in need of steady, capable, leadership. And that’s the kind of leadership that Vice President Biden will offer the country."

The official, taking aim at President Trump, said that Biden's "comments this week have been in stark contrast I think to the confusion and at times outright false information that’s coming from this White House."

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The debate originally was supposed to be in Arizona – one of the four states holding primaries on Tuesday. As concerns over the spreading virus magnified early last week, officials scrapped the live audience for the debate. And, a few days, later the Democratic National Committee announced that the debate would take place in a television studio in the nation’s capital, to cut down on travel for the candidates and others involved in the showdown.

Meredith Kelly, who worked for Senate Minority Leader Chuck Schumer, D-N.Y., and Sen. Kirsten Gillibrand, D-N.Y., before running communications for Gillibrand’s 2020 presidential campaign, noted that for the candidates, “there won’t be feedback from the audience... you’re kind of operating in a vacuum.”

But, Kelly pointed out that the candidates were “still speaking directly to people at home who are going to be more captivated by a debate because they’re isolated in their homes.”

She stressed, “Television is an even more powerful tool when people are sitting at home with very little connections to the outside world otherwise.”

The Sanders campaign acknowledged the uphill climb it’s faced to turn things around and capture the nomination. But, it’s unclear how aggressive the senator will be in taking on Biden.

Sanders seemed hesitant to criticize Biden forcefully when he spoke on Wednesday – his first comments since taking a shellacking from Biden in states including Michigan, Missouri and Mississippi a day earlier.

But, on Saturday evening, that hesitancy disappeared. Sanders was clear that he would try to force Biden to answer his questions “whether or not the moderators, I should tell you, are interested in these issues.”

Sanders elaborated, “I’m going to ask Joe Biden -- I mean, Joe has been part of the establishment for a very long time. Joe, what role have you played in trying to make sure that we end this massive level of income in wealth inequality, where three people own more wealth than the bottom half of America? Now, to me, that is obscene.”

And, in his email Sunday, Sanders pointed out that “the coronavirus crisis is highlighting how stark some” of the policy differences between him and Biden “actually are.”

Biden, trying to bring the nomination race to an end, unite the party and win the support of Sanders and his legions of younger supporters, appeared to take a step toward embracing more progressive positions.

On Friday night, Biden backed the bankruptcy plan of former 2020 rival Sen. Elizabeth Warren of Massachusetts.

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“I’m going to endorse -- I’ve endorsed -- Elizabeth Warren’s bankruptcy proposal, which in fact goes further, allows for student debt to be relieved in bankruptcy, provides for a whole range of other issues that allows us to, in fact, impact on how people are dealing with their circumstances,” Biden explained in a virtual town hall with Illinois voters. “There’s a whole range of things we agree on."

Four hours before the debate, a senior Biden campaign official announced that the former vice president took a second step in embracing a progressive policy pushed by Sanders and Warren by "adopting a policy to make public colleges and universities tuition free for all students who family incomes are below $125,000."

Sanders slapped back - saying Biden's step didn't go far enough.

"It's great that Joe Biden is now supporting a position that was in the Democratic platform four years ago. Now we have to go much further," Sanders stressed in a statement.

If Sanders doesn’t deliver a knock-out blow to Biden on Sunday night, which likely would spell the end of his second White House bid, nudging the former vice president to the left may be his consolation prize.