During oral arguments, two of the three judges expressed some openness to the argument from the states. Of course, any ruling that invalidated the health law would be appealed to the Supreme Court, where the five justices who voted to uphold it in 2012 remain.

Setting aside the politics of the upheaval, the mechanics are not obvious, and could cause policymakers enormous headaches. Here are some provisions that would be particularly hard to make disappear or untangle from the health system.

Consumer protections for insurance customers

Protections for pre-existing conditions weren’t the only rules for insurers established under Obamacare. Without it, health plans could also begin limiting the total amount of financial protection they offer customers, increasing deductibles, charging higher prices to older customers, and dropping expensive categories of benefits, like prescription drugs.

Authorization for the Indian Health Service

The health law included a permanent legal authorization for the Indian Health Service, which runs hospitals, employs physicians and provides health services to more than two million Native Americans. Repealing the law would invalidate the legal basis for that program.

Biosimilar drugs

The health law enabled the Food and Drug Administration to consider and approve biosimilars, drugs akin to generic versions of biologic drugs, which can’t be copied as precisely as typical drugs. Twenty-one biosimilar medications have been approved by the F.D.A. with the new process. Eliminating the health law could jeopardize those approvals and discourage drugmakers from investing in new biosimilar drugs.