Democratic frontrunner Hillary Clinton has been steadfast in her support of the Affordable Care Act and has dismissed Sen. Bernie Sanders’s call for a new national program that would guarantee all Americans health care.

However, Clinton is beginning to feel pressure on health care reform not only from Sanders, her long-shot rival for the nomination, but also from some Democratic lawmakers, rank and file Democrats and health care professionals who say she should be more open to changes to address major shortcomings in Obamacare.

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The latest Kaiser Health Tracking poll shows growing disapproval of the Affordable Care Act, with 49 percent of adults having an unfavorable view and only 38 percent showing support. The share of Democrats voicing unfavorable opinions increased by six percentage points over the past month.

This week more than 2,000 physicians announced their support for a single-payer national health care system similar to Sanders’ call for “Medicare for all” – one that is styled after government health insurance programs in Canada and Europe.

Those doctors complained in an editorial and paper published in the American Journal of Public Health about “persistent shortcomings” in the current health insurance system. The large group of physicians – many of them Sanders supporters -- warned about the risks of continuing with a system that will leave millions uninsured indefinitely while saddling more than 12 million insured Americans with rising premiums.

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The Washington Post, which first reported the editorial, said the idea grew out of discussions in late 2014 involving a smaller group of physicians assessing the impact of Obamacare. The doctors found the program sorely lacking due to the millions of Americans who were still uninsured.

Meanwhile, the Associated Press is reporting that more Democrats are having doubts about Obamacare, especially in the wake of disappointing enrollment figures and complaints about soaring premiums and out-of-pocket costs. Rep. Jim McDermott (D-Wash), one of the most liberal members of the House and a Sanders supporter, told the AP that the healthcare law was a good start "but it doesn't do all the things we need to have done.”

"I think frankly Bernie's campaign has stirred it up," he added.

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The growing Democratic unease with Obamacare comes at a time when some major insurers are reassessing their commitment to the program or have begun to bail out. Nearly half of the roughly two-dozen non-profit Obamacare insurance co-ops have closed their doors in the past year.

UnitedHealth, the nation’s largest health insurance company, recently revealed that it will withdraw from Obamacare insurance exchanges in at least 26 of 34 states where it is currently operating. This week, Humana announced that it will no longer sell insurance on the exchanges in Alabama and Virginia beginning in 2017.

While the Obama administration insists that the federally subsidized health insurance program remains sound, some Democrats are concerned that a new round of premium increases for 2017 will be announced close to the November presidential and congressional election.

Growing Democratic disenchantment with President Obama’s signature health care plan could pose some problems for Clinton, who has been one of the program’s leading champions in the face of Republican calls for repealing and replacing it.

Clinton said she was open to changes and improvements in the law even before she formally announced for president last summer. However, she has been highly critical of Sanders’s call for a multi-trillion-dollar national health care plan funded by major tax hikes. The more pragmatic Clinton has dismissed Sanders’ plan as “wishful thinking” and a threat to the existing health insurance.

Related: UnitedHealth Makes Good on Threat to Pull Out of Obamacare

“I don’t want to rip away the security that people finally have,” she has said. There are plenty of things that could be done to strengthen Obamacare and expand coverage beyond the 90 percent of Americans who are currently insured, Clinton says. But attempting to reopen the health care debate and promote a whole new system of government-provided health insurance would provide GOP lawmakers with an opening to undermine the existing program, she argues.

“I believe that she has been in this lane for as long as the Affordable Care Act has been there and continues to emphasize what needs to be done, with that same commitment to get everybody covered and getting costs under control,” Judith M. Feder, a health care expert and professor with the Georgetown University Public Policy Institute, said in an interview on Friday. “She has been on top of the need to strengthen the Affordable Care Act for some time.”

Related: UnitedHealth CEO: Joining Obamacare Was a ‘Bad Decision’

Recently Clinton has begun to do more to highlight changes she would make in ACA coverage if she were elected president, in order to expand coverage, control costs of premiums and prescription drugs, and encourage states to experiment with government-sponsored alternatives to private health insurance, according to media reports.

In building on the ACA, her campaign states, Clinton will make premiums more affordable, reduce out-of-pocket expenses for consumers purchasing private insurance on the exchanges, and provide more in federal tax credits to lessen costs. She also would support new incentives to encourage all states to expand Medicaid coverage to low income families.

And she would fix the so-called “family glitch” so that families can obtain Obamacare coverage when their employers’ family premium is too expensive.