President Barack Obama warned against repealing the Affordable Care Act (ACA) in a New England Journal of Medicine article, saying without a replacement ready to take its place, the result “could slowly bleed the healthcare system that all of us depend on.”

While the essay repeated many of Obama’s defenses of the law, such as how it’s slowed the growth of healthcare costs and dropped uninsured rates to record lows, he also criticized the current strategy of repealing the law while supposedly delaying the effects until a Republican-backed replacement plan is ready. No such plan has been introduced or been subject to a cost analysis, Obama said, a different route than the debate which took place before the ACA was passed.

“If a repeal with a delay is enacted, the health care system will be standing on the edge of a cliff, resulting in uncertainty and, in some cases, harm beginning immediately,” Obama wrote.

The first dominoes to fall, Obama said, would be insurance plans on the ACA exchanges, on which insurers could offer only significantly more expensive coverage or leave altogether.

Hospitals and providers would also be affected, as care coordination efforts slow and uncompensated care costs quickly rise.

“’Repeal and replace’ is a deceptively catchy phrase,” he wrote. “The truth is that health care reform is complex, with many interlocking pieces, so that undoing some of it may undo all of it.”

One of those interlocking pieces is the individual mandate requiring people to have coverage or face a tax penalty. That provision is one of the least popular aspects of the ACA, but without it, Obama wrote it would be very difficult to keep more popular parts, like coverage for people with preexisting conditions.

The big risk with a “repeal and delay” strategy, he wrote, is that the replacement plan never materializes, leaving more people uninsured than prior to the ACA and creating an even greater burden on hospitals for uncompensated care.

In his conclusion, Obama said Congress should look to the two-party talks which crafted the Medicare Access and CHIP Reauthorization Act (MACRA) as a guide in overhauling or replacing the ACA.

“I will applaud legislation that improves Americans’ care, but Republicans should identify improvements and explain their plan from the start—they owe the American people nothing less,” Obama wrote, adding later, “Policymakers should therefore abide by the physician’s oath: ‘first, do no harm.’”

This was the second time Obama had contributed to a medical journal in the past year. In July 2016, he published a peer-reviewed article in JAMA which also defended the ACA and called for the “public option” to be reconsidered.