Bernie Sanders has conceded that he was losing the nominating contest to former vice-president Joe Biden and had failed to persuade Democrats that he was more electable.
But he also made clear that he was staying in the race, at least for a few more days, determined to meet Mr Biden in a one-on-one debate on Sunday that he said would expose important distinctions between their records.
He even listed a number of challenges he said he would issue to Mr Biden, including how he would fight poverty and how he would handle healthcare.
And he issued a warning to party leaders not to take lightly the resonance of his sweeping liberal ideas or his dominance among younger voters - showing how it is in his and the party's interest to forge the common ground that eluded Democrats after his loss to Hillary Clinton four years ago.
"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," Mr Sanders said.
"You cannot simply be satisfied by winning the votes of older people," he added.
In effect, Mr Sanders was signalling that although Mr Biden's advantage in the race might ultimately prove too difficult to overcome - he notably did not vow to run against him through all 50 state contests as he did against Ms Clinton in 2016 - he intends to pressure Mr Biden to adopt more-liberal positions and has no intention of quietly stepping aside before getting a chance to take him on directly.
"Bernie Sanders won so much support on the left because of his vision, and today he offered Joe Biden a blueprint of things he needs to do to bring over part of the Sanders coalition," said Rebecca Katz, a liberal strategist.
"He's basically telling Biden what the olive branch should be."
Mr Sanders's statement was a wide-ranging recitation of priorities rather than a bill of policy particulars, but it suggested that he could be open to bringing his passionate liberal supporters into the Democratic fold and setting the stage for a highly unusual debate on Sunday that might unfold more like a public negotiation.
That presents Mr Biden, a centrist who has rejected many of Mr Sanders's left-leaning policy ideas, with the delicate challenge of appearing open to the proposals while staying true to his own principles.
That conundrum will be on vivid display at the debate, since Mr Biden has not shown himself to be the most nuanced speaker or nimble debater.
At the same time, Mr Sanders's eagerness for a renewed fight against Mr Biden threatens to complicate Democratic hopes of fostering unity after a bruising stretch.
With the once-sprawling field culling quickly in recent weeks and Mr Biden emerging as the likely nominee in the minds of many Democrats, some are hoping that the former vice-president's critics are ready to lay down their arms now and join the fight against President Donald Trump.
"That's a huge mistake to think that basically he's just not in it to win it," said RoseAnn DeMoro, a long-time Sanders friend and former union head, who said this is an "extremely volatile" time in politics.
The senator spoke on Wednesday from a hotel ballroom in Burlington, Vermont, where he had been hunkering down since scuttling a planned rally in Cleveland on Tuesday over concerns about the spread of the coronavirus.
After making brief remarks to reporters, Mr Sanders left without taking questions. He was to appear on 'The Tonight Show Starring Jimmy Fallon' later in the day.
Less than 24 hours earlier, Mr Biden had extended his lead over Mr Sanders in the all-important race for delegates to the Democrats' July nominating convention, notching big wins in Michigan, Missouri, Mississippi and Idaho. Mr Sanders won North Dakota, while Washington state remained too close to call.
Mr Sanders, delivering his first public comments since the losses, didn't try to sugar-coat his humbling defeats.
"Last night was obviously not a good night for our campaign from a delegate point of view," he said.
But he cited a silver lining: his dominance among young voters and the popularity of his policy ideas. (© 2020, The Washington Post)
US Presidential Election 2020
US Presidential candidate Joe Biden has scored decisive primary victories in four states on Tuesday, taking a big step toward the Democratic Party's presidential nomination and casting doubt on the future of rival Bernie Sanders' fading White House bid.