Burlington, Vermont (CNN) A defiant Sen. Bernie Sanders vowed to stay in the presidential race on Wednesday after a series of defeats that have left him trailing rival Joe Biden in the delegate race, pointing to his appeal with younger voters and previewing the lines of argument he'll use against the former vice president on Sunday at a CNN-Univision debate.

"Last night obviously was not a good night for our campaign from a delegate point of view," Sanders said, listing his his losses in the Michigan, Missouri, Mississippi and Idaho primaries, before noting a victory in North Dakota and reminding viewers of his lead in Washington state.

Sanders' decision to continue his campaign despite the growing odds against him is likely to anger Democrats outside progressive circles, who on Tuesday night began to openly clamor for a quick end to the contest. Biden was largely deferential in his speech and appeared to offer Sanders an off-ramp. But the Vermont senator, after a night of deliberations with his innermost circle, opted to fight on -- and make his case at least one more time to Democratic voters.

Standing in front of a line of American flags and a deep blue curtain, Sanders acknowledged his delegate math crunch before he dug deeper into the numbers and warned that Biden's failure so far to win over young voters could be damaging in November and beyond. He did not, however, criticize Biden over his Iraq War vote or record on Social Security. In dropping those lines of attack, Sanders offered some insight into his goals going forward: to push his own agenda and perhaps secure some commitments from Biden, not tear him down.

"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, before conceding that he was doing just as poorly with older voters.

Sanders, in unusually frank and analytical terms, also discussed his failure so far to convince Democrats he would stand the best chance of defeating President Donald Trump in November.

"While our campaign has won the ideological debate, we are losing the debate over electability. I cannot tell you how many people our campaign has spoke to and 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,'" 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."

The key decision point for Sanders appeared to be the opportunity to stand across from Biden on the debate stage, a one-on-one he has longed for since the campaign began more than a year ago.

Sanders explained his decision, saying he would use the debate to address those electability concerns and, potentially, the trajectory of the race, by peppering "my friend Joe Biden" with a series of questions over his commitment to addressing the issues central to Sanders' agenda.

Among them, the Vermont senator said, was a challenge to Biden on Sanders' signature issue: health care.

"Joe, what are you going to end the absurdity of the United States of America being the only major country on earth where health care is not a human right?" Sanders said, previewing his line of attack. "Are you really going to veto a 'Medicare for All' bill if it is passed in Congress?"

Sanders' "questions" also touched on issues around medical debt, climate change and economic inequality.

A Sanders aide told CNN late Tuesday night that the Vermont senator planned to stay in the race at least through Sunday's debate in Arizona, but as the hours passed and Sanders' team remained publicly silent, speculation began to swirl that he would drop out of the race on Wednesday afternoon.

By fighting on, Sanders will get the face-off with Biden that he has long desired. But it comes now with his campaign on the ropes following Biden's remarkable revival, which began with a sweeping victory in South Carolina and built over the subsequent 10 days as the moderate party leaders and Democratic voters coalesced behind his campaign.

His decision, though, was greeted happily by some leading progressives, who echoed Sanders' focus on Biden's struggles with younger voters.

"I hear people talk all the time about Biden putting together the Obama coalition. Well, the Obama coalition included people under the age of 30, and Biden is losing them by like 60% some cases," said Alexandra Rojas, the executive director of Justice Democrats, a progressive group that endorsed Sanders. "A debate between Bernie Sanders and Joe Biden really, really matters, especially at a time when that key demographic shouldn't be dismissed ... Millions of voters that have voted for Bernie Sanders, especially young people fighting for our future. We deserve to hear that contrast and continue the contest."

Adam Green, the co-founder of the Progressive Change Campaign Committee, which endorsed Warren in 2020 before offering some qualified support to Sanders after she dropped out, said even before the announcement that he and others were hoping to see a debate.

"Not because it will likely change the outcome, but because in order to not change the outcome Biden has to withstand one-on-one scrutiny similar to a debate with Trump -- and he would likely need to cement some popular progressive positions that Bernie challenges him on," Green said. "All of that makes us more likely to defeat Trump."

But before all that, Sanders will resume semi-normal campaign activities on Wednesday night as he heads to New York to join Jimmy Fallon on "The Tonight Show."

This story has been updated with additional developments Wednesday.