BURLINGTON, Vt. — Senator Bernie Sanders on Wednesday made a defiant case for his liberal policy agenda despite suffering big losses in the Democratic primaries this week, saying he planned to continue his bid for the presidency. But he acknowledged that he was “losing the debate over electability” to his rival, former Vice President Joseph R. Biden Jr.

Mr. Sanders vowed to participate in the scheduled debate on Sunday, and asserted that “a strong majority” of Democrats supported his progressive causes, even as his path to the nomination looks increasingly narrow. Still, as he addressed reporters at an afternoon news conference in Burlington, he did not directly attack Mr. Biden or vow to carry his fight to the end, instead signaling he was ready for a de-escalation in their rivalry.

In one striking sequence that highlighted his ideological resolve, Mr. Sanders ticked off a list of policy issues and challenged Mr. Biden to explain to the American people how he would address them — a series of questions that could be seen as an opening gambit for a list of concessions he would seek from Mr. Biden if he were to drop out of the race.