In one of the most heated exchanges over health care during the first of this week’s Democratic presidential debates, Montana Gov. Steve Bullock derided the idea of Medicare-for-all as “wish-list economics” that “rips away quality health care for individuals.”

“We can get there with a public option,” Bullock declared, using shorthand for letting people buy into a government health plan.

That endorsement from a candidate positioned toward the right in the crowded Democratic field reflects a remarkable migration: The notion of a government health plan that many Americans could buy into has gone from being a polarizing liberal wish to a centrist policy favorite within the party.

“It would be an evolution, not a revolution,” former Colorado governor John Hickenlooper said Tuesday night as he, too, told Americans they should have a choice between private and public insurance.

Some version of a public option has emerged as the dominant alternative to left-wing proposals, championed by Sens. Bernie Sanders (Vt.) and Elizabeth Warren (Mass.), to convert the entire U.S. health-care system to a single, government-paid model — dubbed Medicare-for-all by Sanders who has long made the idea central to his political identity.

Among the 10 Democrats on the debate stage Tuesday night, Sen. Amy Klobuchar (Minn.) and South Bend, Ind., Mayor Pete Buttigieg, also said they favor a public option, while former representative John Delaney (Md.) has proposed putting everyone into a government plan, except for those who prefer a tax credit to buy private insurance.

Among the candidates taking part in Wednesday night’s debate, former vice president Joe Biden has said he would add a public insurance alternative to the Affordable Care Act — an idea initially backed by President Barack Obama but one that could not pass the Democratic-controlled Senate. In a new plan this week, Sen. Kamala D. Harris (Calif.) proposes giving Americans a choice between a government plan and heavily regulated private insurance.

The concept of a public insurance option has a decades-long trail in Democratic thought. But wariness within the party, as well as among Republicans, has repeatedly led to the failure of even more modest versions than those espoused by current presidential candidates.

As far back as the 1930s, Democratic presidents have considered ways the government could play a role in guaranteeing Americans affordable coverage. In 1965, when the government created Medicare as federal insurance for people 65 and older and those with disabilities, “all the people who were involved . . . always saw this as a first step that would be built on going forward,” said John Rother, a longtime health policy expert who is president of the National Coalition on Health Care, a broad group working to improve the health-care system.

After the failure of President Bill Clinton’s health-care efforts, he pivoted to work on expanding coverage for groups that especially needed help. His last years in office, Clinton twice proposed a Medicare buy-in that would have allowed some people ages 55 to 64 to pay to join the program.

“It was definitely seen as a very progressive policy at the time, and we did not have uniform support for it,” recalled Dan Mendelson, who specialized in health care for Clinton’s Office of Management and Budget.

The proposals drew little support in the Republican-led Congress of the time. But letting people nearing retirement into Medicare resurfaced in then-Vice President Al Gore’s health plan for his 2000 presidential campaign.

Eight years later, Barack Obama included a public option as part of his health plan in his first presidential campaign. As Congress debated the ACA the following year, the House passed a version that contained a public option, but the idea died in the Senate.

On the Senate floor, some conservative Democrats opposed attempts to include the idea. Then-Sen. Joe Lieberman (Conn.), who had recently switched from a Democrat to an Independent, threatened a filibuster if the legislation contained the public insurance option.

Nearly a decade later, health policy specialists say attitudes have shifted significantly among Democratic politicians and American consumers.

Mendelson, founder of Avalere Health, a Washington-based consulting firm, recalled that when Clinton proposed the Medicare buy-in, many fiscal conservatives, including Democrats, were more focused on the program’s financial health and reluctant to make any changes that might cause the trust fund that pays for hospital services to run out of money more quickly.

“There isn’t as much concern now with either party with the fiscal situation,” Mendelson said, noting that leaders of both parties endorsed a recent budget plan that would raise the federal debt limit.

Most Democratic presidential candidates supporting a public option have not provided details, including whether the government would subsidize such insurance and how much that plan would pay doctors, hospitals or other providers of care. Still, in a climate less focused on federal deficits, Mendelson said, “it becomes a more viable plan.”

Chris Jennings, a Democratic consultant who held senior health policy roles in the Clinton and Obama White Houses, said the idea of letting Americans buy public insurance is intended to address the concerns of a growing cadre of consumers who are insured but face escalating out-of-pocket costs.

Americans who can’t afford insulin “can’t wait for a revolution,” Bullock said Tuesday. “Their problems are in the here and now.”

According to Jennings, “the biggest driver in health care today is that two-pronged dynamic duo of cost and complexity” of private insurance. Talk of a public option “at least offers the rhetorical benefit of keeping the private plans honest so there is more competition and more affordable options. . . . That’s the promise of it.”

Julie Tate contributed to this report.