“If Democrats can further advance toward near-universal coverage without the life-or-death struggles of Medicare for All,” Mr. McDonough wrote, “they might just achieve meaningful and historic progress even as they preserve the political capital to make progress on other compelling and urgent policy needs.”

Mr. McDonough also pointed out that the landmark coverage expansions in 1965 (which also created Medicaid, but for a very limited group at the time) and in 2010 with the Affordable Care Act were passed not merely by a Democratic-controlled Congress, but also with Democratic supermajorities in the Senate.

“There’s no prospect of having majorities like that,” said Paul Starr, a professor of sociology and public affairs at Princeton University. “It’s not going to happen.”

Even if the rules were changed to get rid of the filibuster, making it possible to pass major legislation with only 50 Senate votes, “there is not any guarantee that the 51st Democrat would be willing to support Medicare for all or anything close to it,” said Mark Peterson, a professor of public policy, political science and law at the University of California, Los Angeles.

Rep. Pramila Jayapal, Democrat of Washington, who wrote the House Medicare for all bill, said in an interview that she was not discouraged by the math, noting that since she introduced her bill a year ago, a dozen more members had signed on as co-sponsors, House committees had held four hearings on it, and coalitions representing people of color, labor unions and businesses had begun lobbying for the bill.

“A lot of the members I speak to that aren’t on the bill, I actually believe they would like to be on the bill but think, ‘I don’t know if it’s politically good for me,’” Ms. Jayapal said. “That would fundamentally change if Bernie were to be elected president.”

Proponents of Medicare for all like to cite some polls that suggest there is strong support for the idea. But Mollyann Brodie, who oversees public opinion research at the Kaiser Family Foundation, says only a minority of Democrats are solely in favor of a sweeping Medicare for all plan, compared to a majority who support offering the option of buying a Medicare-like plan or Medicare for all as a way to address high costs and the challenges of getting care.