When Sen. Kamala Harris (D-CA) unveiled her long-awaited health-care plan on Monday, some of her opponents criticized it for going too far while others said it lacked ambition. But there was one criticism that seemed to unite both sides: Its long phase-in window would leave the proposal deeply vulnerable for whoever might succeed Harris’ presidency.

Harris’ “Medicare for All” plan differs from Sen. Bernie Sanders’ (I-VT) legislation in the Senate—which Harris has also supported—in that it allows for private insurers to offer plans in the Medicare system that adhere to strict regulatory requirements. Her plan also seeks to avoid hitting middle class families by setting the threshold for those who would be taxed at $100,000 per household. And then there is the phase-in. Under Harris’ proposal the full implementation of the law would only come after a 10-year phase-in (compared to Sanders’ proposed four-year glide path) in order to alleviate concern over quick and dramatic market changes.

That latter point was not the main topic of contention on Monday, as health-care wonks debated the specifics of Harris’ proposal. But it is one that left many Democrats questioning the viability of her plan.

“If President Obama had passed a health-care bill that called for a 10-year horizon, he would have left it to Donald Trump to execute,” Faiz Shakir, Sanders’ campaign manager told The Daily Beast. “Just think about the opportunities you give to the insurance industry and political opponents to water down and chip away and morph and cut the plan you’ve passed,” he said. “That is our historical experience with Obamacare.”

Shakir was referencing how easily the Affordable Care Act has came under attack when Obama left office—both through Trump’s regulatory actions and legislative efforts by the Republican-led Congress. Though Obamacare remains standing, many of its provisions have been scrapped, weakened, or dramatically curtailed. Veterans of the past administration recognize that had Obama lost re-election to Mitt Romney in 2012, the law may never have been implemented at all.

“Yes, you would like to do as much as one can within the first four years and the first eight years,” said Phil Schiliro, director of legislative affairs for the Obama administration from 2009 to 2011, when asked about Harris’ 10-year phase-in. “There is a theory that once popular programs start they take on a life of their own and it becomes difficult for subsequent administrations to do them."

Another Obama veteran who worked on the ACA conceded that they were naive in their assumption that Republicans would simply grow accustomed to Obamacare overtime, even if they tinkered around the edges. That GOP lawmakers didn’t, the veteran said, should have animated how Harris structured her own plan.

“The dangers are obvious on their face to everyone who has watched what has happened with the Trump administration and the Affordable Care Act. You expect the other party to act in a rational way with legislation that advances the public good or is reasonably popular. But that doesn’t happen,” the Obama veteran said. “So if you go beyond even the four-year mark and don’t have implementation down, it is tough to take that completely seriously.”

Even former Vice President Joe Biden’s campaign, which has proposed a health-care plan including a public-insurance option but no promise of Medicare for All, sought to portray the proposed timeline from Harris as unrealistic.

“This new, have-it-every-which-way approach pushes the extremely challenging implementation of the Medicare for All part of this plan ten years into the future, meaning it would not occur on the watch of even a two-term administration,” Kate Bedingfield, deputy Biden campaign manager said on Monday.

The pushback from Obama veterans and fellow campaigns helped illustrate a truism of the politics of health care—that things get incredibly complicated once you are the one offering a specific set of reforms. Harris’ positions have, at times, been unclear throughout the primary. During the first two-night Democratic debate last month, she was one of four candidates to raise her hand when asked if she favored the elimination of private insurance. But afterward, she said she misunderstood the question and thought it pertained to whether or not she herself would give up private insurance.

“No issue is more personal to Americans than their health care, so as Kamala implements a plan like this, she thinks it’s important to get it right,” Ian Sams, national press secretary for Harris said in a statement. “Americans need to have confidence in the transition, and during this transition, there would be an immediate Medicare buy-in, as well as automatic enrollment for the uninsured and newborns in a public Medicare plan. Ten years is a responsible transition period to ensure that every American can move onto the Medicare system smoothly.”

In Harris' Medium post explaining the policy, the senator addressed the issue of a 10-year phase in by both noting the positives that it provided—mainly, more time for consumers and stakeholders to adjust to the changes—and by explaining all the work that would be done in the interim. “[W]e will automatically enroll newborns and the uninsured into this new and improved Medicare system, give all doctors time to get into the system, and provide a commonsense path for employers, employees, the underinsured, and others on federally-designated programs, such as Medicaid or the Affordable Care Act exchanges, to transition.”

Harris argued that the longer window would decrease the overall cost of the proposal—though that largely will be marginal over an extended time period—and said that once the plan passes, all Americans will have the opportunity to immediately buy into Medicare. “Right away, it will lower costs and give us a baseline plan as we transition to Medicare for All,” Harris wrote.

Supporters of Harris’ plan, including former Secretary of Health and Human Services Kathleen Sebelius, said that one of the reasons a 10-year phase in wouldn’t leave the proposal vulnerable to Harris’ successor is that she was expanding a known product. Where the Affordable Care Act was—outside of the Medicaid expansion—the creation of a newly-regulated private marketplace, Medicare is a product the public likes and one which would be harder to demonize.

“One of our difficulties was there were really not the full array of benefits until you got to year four,” she told The Daily Beast. “And that created lots of issues and allowed lots of people to say ‘Well this will happen and that will happen, and there’ll be death panels.’ There was no way to really combat the kind of false narratives.”

But congressional veterans, including those who were in the trenches of the Obamacare fight, aren’t as Pollyannaish, in part because the bill they helped pass in 2010 was structured on conservative health-care policy principles, and Republicans called it a government takeover regardless.

“I’ll wait to hear more details, but it sounds to me like someone is just kicking the can down the road in hopes that another president will pick up the pieces,” said Jim Manley, a 21-year veteran of the Senate who helped get the ACA passed. “We struggled with this while crafting Obamacare. You needed some time for the bill to be implemented but you also need to put in some benefits as quickly as possible. You can’t possibly depend on someone else to follow through on what you proposed.”

Already, there is some evidence that Harris would face sustained pushback for the duration of her law’s implementation and not just from opposing lawmakers. The insurance industry did not signal openness to her proposal on Monday, and has even expressed reticence with significantly more modest options presented in the Democratic primary.

“We agree that every American deserves affordable coverage choices. But like other plans, this proposal puts us on a slippery slope to Medicare for All, and does nothing to address our top health-care challenge: out-of-control costs,” Kristine Grow, a spokesperson for America’s Health Insurance Plans, a trade and advocacy association of health insurance companies told The Daily Beast. “People will have to pay more, to wait longer, for worse care. Our top health-care priority needs to be making health care more affordable for all Americans.”

Update 8:41 AM: This story was updated to include comment from Harris' team.