Addressing the media in Burlington, Vermont, Sanders acknowledged Biden's front-runner status and focused his remarks on issues he plans to press the former vice president on during Sunday night's debate.
"Last night, obviously, was not a good night for our campaign from a delegate point of view," he said, adding, "While we are currently losing the delegate count ... we are strongly winning in two enormously important areas which will determine the future of our country."
He pointed to the popularity of his agenda in some polling, including "Medicare for All," as well as his strong support among young voters.
"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," Sanders said. "And you must speak to the issues of concern to them. You cannot simply be satisfied by winning the votes of people who are older."
But while Sanders said he is winning on those issues, he said he is "losing the debate over electability," saying he and his campaign have heard from voters across the country who like his platform but feel Biden has a better chance of taking on President Donald Trump.
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Sanders said he looks forward to debating Biden on Sunday, pledging to ask what Biden would do for Americans struggling with health care costs, whether he'd veto a Medicare for All bill and how he'd change the American energy system, among other topics.
"Donald Trump must be defeated, and I will do everything in my power to make that happen," Sanders said, adding that, on Sunday, "the American people will have the opportunity to see which candidate is best positioned to accomplish that goal."
Sanders came out of the gates strong in Iowa and New Hampshire, followed by a resounding victory in Nevada. His fortunes began to change after the fourth primary contest in South Carolina.
Biden picked up a commanding win there last month, which led to the abrupt exits of Amy Klobuchar and Pete Buttigieg, who promptly endorsed him. Biden surged on Super Tuesday, jumping ahead of Sanders in NBC News' national delegate count and racking up endorsements from more former rivals, including Kamala Harris, Cory Booker and Mike Bloomberg.
Then, on Tuesday, Biden picked up a big victory in Michigan — a state that had gone to Sanders by a slim margin in 2016 and was viewed as make or break for his candidacy. Biden now leads Sanders in the NBC News delegate count, 837 to 698.
Sanders went from leading Biden by 11.5 points in the RealClearPolitics average of national polls one day before South Carolina to now trailing him by 17.4 points.
Next up on the primary slate is a series of delegate-rich contests in Florida, Arizona, Ohio and Illinois. Sanders lost all four to Hillary Clinton in 2016.