(CNN) Congressional Democrats are locking arms to protect the Affordable Care Act -- the party's signature achievement during Barack Obama's presidency -- as President Donald Trump renews his efforts to repeal it.

With one prominent exception.

In media interviews last week, Sen. Bernie Sanders -- among the front-runners for the party's 2020 presidential nomination -- pointedly refused to endorse legislation that House Democrats had introduced to expand the ACA and to block Trump administration initiatives designed to weaken some of its core consumer protections. Instead, Sanders underscored his support for replacing the ACA with "Medicare for All" legislation, in which government would become the "single payer" for virtually all health care services.

"The incremental reform that I support," Sanders told The New York Times , "is phasing in 'Medicare for All.' " Even when Sanders later wrote on Twitter that Democrats "must defend the ACA from Trump's assault," he did not specifically endorse the House bill and sharply added that "protecting the ACA will not fully solve the health care crisis."

Sanders' cool posture toward the House legislation crystallizes the critical choice Democrats face on health care. On Tuesday, Trump tweeted that Republicans would seek to replace the ACA after the 2020 election, thus increasing the chances voters view the race as a referendum on whether to rescind the law.

With Trump again emphasizing his determination to undo the ACA -- both by backing litigation that would invalidate the entire law and by proposing to replace it with block grants for states in his latest budget -- Democrats must decide how to respond. The choice is whether to center the party's 2020 health care message on defending and reinforcing the ACA or to offer their own fundamental change by proposing to replace the law with a single-payer plan that would eliminate virtually all private health insurance.

That choice, in turn, illuminates the broader crossroads awaiting Democrats in the 2020 election.

More than any of his rivals, Sanders is betting that voters are open to fundamental changes that would vastly expand government's role in the economy. Although the party's center of gravity has undeniably moved left since the Vermont independent's unexpectedly strong showing against Hillary Clinton in the 2016 Democratic presidential primaries, almost all of the other contenders -- with the arguable exception of Sen. Elizabeth Warren of Massachusetts -- are offering more incremental changes that accept greater limits on government, both in cost and degree of control over the private-sector economy.

The congressional Democratic leadership is tilting clearly toward the latter camp, for instance by stressing the legislation to repair the ACA rather than the single-payer proposal that excites the left.

Almost all leading Democrats are now receptive to a greater role for government than in the 1990s, when President Bill Clinton famously declared, "The era of big government is over." Even compared with Obama, most of the 2020 field is offering proposals, on everything from increasing teacher pay (Sen. Kamala Harris of California) to imposing a wealth tax (Warren), that envision a more expansive -- and expensive -- role for Washington in driving economic and social change.

What do the voters want?

But the question of how much change -- and how great a role for government -- the electorate will accept looms as a key dividing line in the 2020 Democratic race. That will be especially true if former Vice President Joe Biden enters the race as expected. With his long roots in more centrist Democratic politics, Biden would likely question whether vanguard liberal goals such as a government takeover of health care, the near-term elimination of fossil fuels, or tuition-free public higher education are electorally feasible, or even desirable from a policy perspective.

An extensive new poll of likely Democratic primary voters released Friday by Third Way, a centrist Democratic group, offers some hints on how this debate may unfold.

The survey, conducted March 7 to 10 by Democratic pollster David Binder, generally found most Democratic voters leaning toward incremental change rather than a radical restructuring of government and the economy. That instinct surfaced across an array of questions:

A 55% majority of Democrats said they preferred a 2020 nominee who "says we should mend, but not end capitalism," while only 23% wanted one who "calls themselves a Democratic socialist." (The rest were unsure.)

59% wanted a candidate who focused "on expanding opportunity to more people and places," compared with 33% who wanted one who "will focus on reducing income inequality." (The rest didn't know).

70% said they preferred a nominee who "appeals to a broad range of voters," while only 22% wanted one who "moves further to the left to generate enthusiasm."

Still, detailed results found a greater constituency for fundamental change among likely Democratic voters younger than 35. Nearly two-fifths of them in the survey, far more than older generations, preferred a candidate who called themselves a Democratic socialist rather than one who wanted to mend capitalism. And the share of younger Democrats who preferred a nominee who moved left to generate enthusiasm, while still a minority, was also higher than for older generations. Those findings help explain why Sanders ran so well among younger voters in 2016, and why polls again show him attracting much more support among younger than older Democrats.

Health care remains the issue where these broader choices are likely to reach the sharpest point for Democrats through the 2020 election.

Single payer legislation in which government would eliminate private health insurance and directly fund a guaranteed package of benefits through tax dollars remains the rallying cry for the left. Four other 2020 presidential contenders -- Sens. Kirsten Gillibrand of New York, Cory Booker of New Jersey, Harris and Warren -- have endorsed Sanders' single payer bill in the Senate. In the House, 105 voting Democrats have endorsed the companion bill from Rep. Pramila Jayapal of Washington state.

But the legislation that House Democratic leaders introduced last week to strengthen the ACA has drawn broader support. As a recent CNN analysis found , the single-payer bill has been endorsed by only two House Democrats from districts that backed Trump over Clinton in 2016 and just seven of the Democrats who captured previously Republican seats last November. By contrast, about two dozen first-term Democrats who flipped Republican seats last fall -- including seven who won in districts Trump had carried -- have already endorsed the bill to stabilize and expand the ACA.

Fixing it vs. starting over

That bill, which would overturn Trump administration actions loosening protections for patients under the ACA and increase subsidies for people to buy insurance through the law, embodies the bread-and-butter focus that most Democrats emphasized on health care in the 2018 midterm elections -- and that many party strategists see as the most appealing course for 2020.

The bill represents a first step toward one future health care path for Democrats: building on existing programs, including the ACA and Medicare. For many Democrats, the next step might be legislation that allows all adults (or those over 50, depending on the proposal) to buy into Medicare while permitting those who want to maintain private health insurance to keep it.

Presidential contender Beto O'Rourke aligned with that option in his kickoff campaign speech Saturday in El Paso, Texas, when he declared: "We can give every American, every business in this country the choice to enroll in Medicare without eliminating plans that many Americans like for their families because those plans work for their families."

Sanders dismisses these alternatives as insufficient because they maintain private health insurance. His coolness toward the new House bill sets him apart: Even Jayapal, the lead sponsor of the House single-payer bill, has backed the bill. So has liberal champion Rep. Alexandria Ocasio-Cortez of New York. Likewise, Harris, who has also called for eventually replacing private health insurance with a single-payer plan, "of course ... supports other efforts to strengthen ACA and expand protections for pre-existing conditions," said Ian Sams, her national press secretary, in an email.

Sanders' position confused many Democratic health care experts and angered party centrists. (Asked for further detail on his view, his Senate office pointed to his tweet last week.)

"This is tone deaf among all but a small slice of Democratic primary voters," said Jim Kessler, Third Way's executive vice president for policy. "With so many people afraid about pre-existing conditions and losing their health care under Trump, there is a moral urgency to be pragmatic while we also try to get to universal coverage. This is precisely why Bernie Sanders has a ceiling (of support) and he just bumped up against it."

Still, the outcome of this debate among Democratic primary voters is far from certain.

Third Way's poll found that the share of Democratic voters who considered "reducing the cost of health care" one of the most important issues for candidates to address was double the percentage that cited passing a single-payer plan. But in the latest monthly poll by the nonpartisan Kaiser Family Foundation , Democratic voters, by a narrow 46% to 44% plurality, said congressional Democrats should focus on passing a single-payer plan rather than "improving and protecting" the ACA.

Democrats under 50 preferred focusing on single-payer by a double-digit margin, while those over 50 narrowly broke toward strengthening the ACA, according to detailed results provided by Kaiser.

When Kaiser last asked that question in January, strengthening the ACA led by a double-digit margin among Democrats overall. The quick shift in the latest survey underscores how fluid opinion on this choice remains -- and how much the debate among Democratic candidates through the coming months may shape perceptions.

Whatever policy Democrats ultimately endorse, Trump has guaranteed a stark contrast in 2020 by renewing his efforts to repeal the ACA. The question is whether Democrats will counter that with proposals to build on the law or offer their own blueprint for uprooting and replacing it, as Sanders is urging.