Underlying the dispute are different assumptions about how single payer would work. How much could we save if the government negotiated directly with drug companies? How much care would patients seek in a world without co-pays?

These aren’t minor details; they get to the heart of the most common attack on single payer — that it’s too costly and too disruptive. Moving to true single payer would effectively end the current role of the private insurance industry, not only getting rid of Obamacare’s exchanges but also the employer-based plans that cover more than half of non-elderly Americans.

Single-payer advocates believe that radical change is necessary, pointing out that the United States spends more on health care than any other wealthy country, with some of the worst health outcomes. But many advanced, industrialized democracies with universal coverage don’t have a pure single-payer system. France, for instance, has health care for all that is largely state-financed, but most people also buy private supplemental coverage.

While “single payer” has become an effective political rallying cry, advocates still need to figure out what it would mean for one of the largest, most complex health care systems in the world. Senator Sanders himself is preparing to introduce a single-payer bill that will be “far more detailed than the campaign plan” and include changes to address cost concerns, said a spokesman, Josh Miller-Lewis.

Mr. Baker believes the top priority is a credible transition plan. “If you just take everyone with employer-provided insurance and put all of them on a public plan, you’re going to freak people out,” he said. He’s interested in reviving the public option — a government-run plan that would compete with private insurance on the exchanges — as well as opening up Medicare or Medicaid to those who want to buy in.

It’s unclear how receptive the base would be to incremental reforms. They could be a reminder of what’s hamstrung Democrats in the past: ceding ground to centrists who insist on largely unobjectionable — and uninspiring — white papers. Jeff Hauser, a progressive strategist, argues that the movement should come before the details. “You don’t build a political coalition around wonks,” he says.

What Democrats need to emphasize above all, Mr. Hauser argues, are big ideas that can energize their supporters. More than anything, “single payer” has become shorthand for the notion that everyone deserves health care. “Health care should be a right. It should never be a privilege,” Senator Kirsten Gillibrand, Democrat of New York, and a potential 2020 presidential candidate, recently asserted. “We should have Medicare for all in this country.”

But the more political headway that single payer makes, the more supporters will need to explain how it could actually work in practice. Otherwise, Democrats risk making the same mistake on health care as Republicans: big promises without a plan to follow through.