“Vice President Biden fought to get the biggest reform to our health care system in a generation done, so insurance companies know where he stands,” said T.J. Ducklo, a spokesman for Mr. Biden’s campaign. “And based on their reaction to his health care plan, we’re not expecting too many contributions.”

Until recent days, Mr. Sanders and Mr. Biden have largely avoided attacking each other; during the first debate, when they were on the same stage, neither markedly sought to highlight their differences. But in seeking to draw an explicit contrast over health care, both candidates have opened a new front in the fight for the Democratic nomination that also underscores the progressive-versus-centrist divide in the party.

The speech Wednesday, like one Mr. Sanders gave last month on his philosophy of democratic socialism, seemed intended to garner maximum news media exposure: He delivered it in Washington, steps away from where many political journalists work, rather than in an early voting state like Iowa or New Hampshire.

Though he referenced some criticisms of Medicare for all, including some offered by Mr. Biden, Mr. Sanders did not underscore the differences between his position and that of other candidates. Nor did he offer much in the way of new details on his plan.

Instead, he provided his well-worn rationale for a single-payer system, saying it would afford families comprehensive insurance coverage and “save lives.”

“Frankly, I am sick and tired of talking to doctors who tell me about the patients who died because they came into their offices too late because they were uninsured or underinsured,” he said.

In response to claims that transitioning to his health care system would not be feasible, he pointed to the passage of Medicare in 1965, a program that he said was similarly disparaged.