Sanders then challenged Biden to prove that he can offer those voters solutions to a host of key policy questions during a televised debate on Sunday in Phoenix, where the two candidates are set to face off one-on-one for the first time. Sanders and his aides have long wanted a one-on-one debate with Biden, believing that such an event could change the dynamics of the race.

"Let me be very frank as to the questions that I will be asking Joe," Sanders said. "Joe, what are you going to do for the 500,000 people who go bankrupt in our country because of medically related debt? And what are you going to do for the working people of this country and small business people who are paying on average 20 percent of their incomes to health care?"

He then reeled off more queries on issues including climate change, college affordability, criminal justice and child poverty, as well as two other frequent talking points for the Sanders campaign: campaign finance and income inequality.

"Joe, importantly, what are you going to do to end the absurdity of billionaires buying elections and the three wealthiest people in America owning more wealth than the bottom half of our people?" Sanders said.

The senator's roughly 10-minute remarks came a day after his path to the nomination narrowed significantly, following state primaries in Idaho, Michigan, Mississippi, Missouri, North Dakota and Washington. Of those nominating contests, Sanders captured only North Dakota, while Washington’s votes are still being tabulated.

Among his other wins, Biden walked away with a decisive victory in Michigan, the biggest delegate prize Tuesday and a critical test of Sanders’ strength, after he beat Hillary Clinton in the state during the 2016 Democratic primary.

But Sanders fell short this time around, failing to muster a much-needed jolt of momentum for his campaign, which has struggled since Biden supplanted him as the party’s frontrunner in last week’s Super Tuesday primaries.

Sanders isn’t one to give up, no matter how much the odds are stacked against him. He did not drop out of the 2016 race after it was mathematically impossible for him to win, nor did he abandon his 2020 bid after a heart attack. Sanders is scheduled to appear on NBC’s "The Tonight Show With Jimmy Fallon" on Wednesday.

It is also possible Sanders sees the debate as a way to push Biden’s platform to the left.

Many progressives, even those who did not support Sanders this year, want to see a debate between Sanders and Biden in order to push Biden toward more left-leaning policies — as well as to test Biden before what is expected to be a brutal general election campaign against President Donald Trump.

The Progressive Change Campaign Committee, a group that is closely aligned with Elizabeth Warren, urged Sanders to go to attend the debate and prevent a “quick Biden coronation.”

Rep. Ayanna Pressley, who endorsed Warren, also tweeted “#KeepTheDebates,” adding: “Because we deserve to have a public conversation about racial justice, the Black white wealth gap, disability justice, immigration, LGBTQ equality, reproductive justice, the Latinx & AAPI communities & so many other priorities that have been left off that stage.”

And Sanders insisted Wednesday that he intends to remain in the race at least long enough to confront Biden in the weekend debate ahead of four more primaries on March 17: Arizona, Florida, Illinois and Ohio.

“Donald Trump must be defeated, and I will do everything in my power to make that happen. On Sunday night in the first one-on-one debate of this campaign, the American people will have the opportunity to see which candidate is best positioned to accomplish that goal," Sanders said at the conclusion of his speech. He did not take questions from reporters.

Sanders is facing intensified calls following Tuesday’s results to exit the primary and follow several of his former rivals in backing Biden.

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Cory Booker, Pete Buttigieg, Kamala Harris, Amy Klobuchar, Beto O’Rourke, Andrew Yang and others have all endorsed the former vice president. Many mainstream Democrats are eager to put an end to months of Democratic infighting and launch the general election campaign against Trump.

But Sanders on Wednesday warned Democrats against the perils of nominating a candidate who fails to galvanize youth turnout.

"Today, I say to the Democratic establishment: In order to win in the future, you need to win the voters who represent the future of our country, and you must speak to the issues of concern to them," Sanders said. "You cannot simply be satisfied by winning the votes of people who are older."

Sanders also spoke to doubts among Democratic voters regarding his ability to vanquish Trump, conceding that "we are losing the debate over electability" to Biden.

"I cannot tell you how many people our campaign has spoken to who have said, and I quote, 'I like what your campaign stands for. I agree with what your campaign stands for. But I'm going to vote for Joe Biden because I think Joe is the best candidate to defeat Donald Trump.' End of quote," Sanders said.

"We have heard that statement all over this country. Needless to say, I strongly disagree with that assertion. But that is what millions of Democrats and independents today believe," he continued. "On Sunday, I very much look forward to the debate in Arizona with my friend, Joe Biden."