WASHINGTON — In April, many of the top Democratic presidential candidates eagerly lined up to cosponsor Bernie Sanders' Medicare for All bill — a vast restructuring of the U.S. health care system that would go far beyond Obamacare.

Sens. Elizabeth Warren, Kamala Harris, Cory Booker and Kirsten Gillibrand, along with Reps. Tim Ryan and Tulsi Gabbard, all co-sponsored the bill with an eye toward the upcoming primaries.

"It was a recognition that the center of gravity in the party has moved in a much more progressive direction," said Jeff Weaver, a senior adviser to Sanders' campaign. "Many candidates wanted to position themselves with the vast majority of Democratic primary voters by supporting real Medicare for All."

But as this week's Democratic debates in Detroit illustrated, many of those initial cosponsors, fearful of blowback from voters — particularly those who have satisfactory private health insurance they're reluctant to give up for something unknown, as they would have to under Sanders' plan — have begun backing away.

Sanders' Medicare for All would expand to all American residents the government-run health insurance program that's covered senior citizens and certain other people for more than 50 years. But as the costs and disruptions of the plan have come into focus, Medicare for All has emerged as the major fault line in the Democratic presidential primary.

Health-care coverage consistently ranks as the top issue for Democrats and helped drive the party's electoral gains in 2018. But polls show that voters harbor deep concerns about the possible disruption from a policy as far-reaching as Sanders has proposed.

The two-night gathering in Detroit made clear that the biggest fight in Democratic politics right now is whether the party should press ahead with remaking the health care system, which accounts for 18% of U.S. gross domestic spending, or instead pursue more limited reforms along the lines of front-runner Joe Biden's proposal to "build on Obamacare," the signature legislative achievement of President Barack Obama.

"At the level of the bumper sticker or talking points, proposals are very popular, but when you start filling in the details and the trade-offs become clear, it becomes more apparent that there are losers as well as winners," said Larry Levitt, executive vice president for health policy at the Kaiser Family Foundation. "At the bumper sticker level, it seems that everyone is a winner."

A PBS NewsHour/Marist poll of U.S. adults conducted July 15-17 highlighted the Democrats' dilemma. It found that while 70% of respondents favor a Medicare for All option, only 41% support doing away with private health insurance. A recent tracking poll from the Kaiser Family Foundation showed overall favorability for a Medicare for All system dropped to 51% in July from 56% in April. It also found that more Democrats, 55%, prefer to expand coverage by bolstering the Affordable Care Act than the 39% who believe in replacing Obamacare with a Medicare for All system.

During the debates, Booker and Gillibrand both dodged questions about the role of private insurance under Medicare for All. Instead, they sought to reframe the conversation about health care in broader terms, drawing a contrast between Democrats and Republicans.

"This pitting progressives against moderates, saying one is unrealistic and the other doesn't care enough, that to me is dividing our party and demoralizing us in face of the real enemy here," Booker said.

Gillibrand said the intraparty squabbling risked "losing the forest through the trees."

Harris, in particular, has struggled to stick to a clear position on Medicare for All. The California senator was the first Democrat to cosponsor Sanders' bill and raised her hand when moderators at the June debates asked which candidates would be comfortable abolishing private insurance. Harris backtracked, saying she'd misheard the question.

"She violated a rule I've promulgated in politics: If you're caught in a bind and can't think of any way out of it, don't say something that no one will believe," said Barney Frank, the former Democratic representative from Massachusetts, who has not endorsed a presidential candidate for 2020.

On July 29, Harris introduced her own less ambitious Medicare for All plan that also allows private insurance coverage and would be phased in over a decade.

Kate Bedingfield, a spokeswoman for former Vice President Joe Biden's campaign, assailed it as a "have-it-every-which-way approach" that would raise middle-class taxes. Biden amplified that charge during the debate by saying, "You can't beat President Trump with double-talk on this plan."