LAS VEGAS --Rank-and-file Democrats appear to be shifting to the middle on health care, worried about what's politically achievable on their party's top 2020 issue.

While "Medicare for All" remains hugely popular, the majority say they'd prefer building on "Obamacare" to expand coverage instead of a new government program replacing America's mix of private and public insurance.

Highlighted by a recent national poll, the shifting views are echoed in interviews with voters and the evolving positions of Democratic presidential candidates. Some have backed away from the government-run plan championed by Sens. Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts that for months had seemed to be gaining momentum.

It could mean trouble for Sanders and his supporters, signaling a limit to how far Democratic voters are willing to move to the left amid doubts Americans would back such dramatic changes to their health care.

"We hear Medicare for All, but I'm not absolutely certain what that means and what that would then mean for me," said Democrat Terrie Dietrich, who lives near Las Vegas. "Does it mean that private insurance is gone forever?"

Dietrich, 74, has Medicare and supplements that with private insurance, an arrangement she said she's pretty comfortable with.

She thinks it's important everyone has health care, not just those who can afford it. She said she would support Medicare for All if it was the only way to achieve that.

But "I don't think we can ever get it passed," Dietrich added.

Erin Cross, her 54-year-old daughter and also a Democrat, said she's uncomfortable with switching to a system in which a government plan is the only choice. She said Democrats won't be able to appeal to Republicans unless they strike a middle ground and allow people to keep their private insurance.

"We've got to get some of these other people, these Republican voters, to come on over just to get rid of Trump," she said.

Democratic presidential candidates also have expressed skepticism.

California Sen. Kamala Harris' new plan would preserve a role for private insurance. New Jersey Sen. Cory Booker is open to step-by-step approaches. Meanwhile, health care moderates including former Vice President Joe Biden have been blunt in criticizing the government-run system envisioned by Sanders.

In Nevada, the early voting swing state testing presidential candidates' appeal to labor and a diverse population, moderate Democrats have won statewide by focusing on health care affordability and preserving protections from President Barack Obama's law.

Nationwide, 55% of Democrats and independents who lean Democratic said in a poll last month they'd prefer building on Obama's Affordable Care Act instead of replacing it with Medicare for All. The survey by the nonpartisan Kaiser Family Foundation found 39% would prefer Medicare for All. Majorities of liberals and moderates concurred.

On a separate question, Democratic support for Medicare for All was a robust 72% in July, but that was down from 80% in April, a drop Kaiser says is statistically significant but not necessarily a definitive downward trend.