But the debate over health care may be unique in its potency. It mirrors a larger struggle among Democrats over how daring their message ought to be, and whether promising to rapidly expand social-welfare programs is the best way to defeat President Trump. In the past, Democrats have tended to nominate relatively moderate candidates, with even nominees like former President Barack Obama espousing platforms far less radical than the one favored by Mr. Sanders.
Polls show that Democratic ideas for expanding government health care are popular, but the key details of a single-payer system can make many voters uneasy.
In the debate over single-payer, several prominent Democratic candidates and officials have endorsed some version of “Medicare for all,” a broad slogan that has been applied to a range of policies that would expand government-backed health care. In the purest conception, favored by Mr. Sanders, it involves the creation of a multitrillion dollar government health plan through which every American would obtain essential coverage. That approach, which is typical across most of the developed world, would guarantee universal coverage for all Americans and reduce health care costs for many, but it has long been seen as politically controversial because it would require so much public funding and would force many people who like their existing health insurance to change plans.
Jared Bernstein, a liberal economist who served in the Obama administration, said the distinctions between Democratic candidates had less to do with where they want to take the country than with how — and how quickly — they aim to get there. Their core priorities were largely identical, Mr. Bernstein said.
“The candidates who are trying to carve out more moderate positions are essentially saying, ‘I think we get from where we are to where we need to go through incremental steps.’ The others are saying, ‘No, we’re into giant steps,’” Mr. Bernstein said. On health care in particular, he said, “The real difference between the candidates on this is whether you believe we get where we need to go on health care through incremental change, or by leapfrogging to something much more universal than we have.”
That Mr. Sanders would be a champion of giant steps is no surprise. His insurgent campaign in 2016 exploded the bounds of traditional debate in Democratic politics and thrust once-remote ideas — like European-style health care and free tuition at public colleges — to the center of debate on the left. He has shown every sign so far of approaching 2020 in a similar spirit, and describes other proposed policy solutions as inadequate half measures.
In his announcement interview, Mr. Sanders offered scant acknowledgment of the practical and political obstacles to implementing his vision. He cast all impediments as the products of a corrupt political system that could be overcome through a mass mobilization of the popular will.