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.