Formal regulations would take time to undo, because they must follow a lengthy process allowing for public comment. But there are several measures Trump could take on day one of his presidency to cripple the law’s effectiveness.

Perhaps Trump’s easiest action—and the one that would produce the largest impact—would be to drop the administration’s appeal of a lawsuit filed by Republican House members in 2014. That suit, House v. Burwell, charged that the Obama administration was unconstitutionally spending money that Congress had not formally appropriated; it was spending funds to reimburse health insurers who were providing coverage to working-poor policyholders—those earning between 100 and 250 percent of the federal poverty line. More than half of people who purchase insurance in the health exchanges get the additional help, which reduces out-of-pocket health spending on deductibles and coinsurance. While that aid for consumers is required under the law, the funding was not specifically included.

Last April, Federal District Court Judge Rosemary Collyer ruled in favor of the House Republicans. “Such an appropriation cannot be inferred,” she wrote of the payments, and insurer “reimbursements without an appropriation thus violates the Constitution.” However, Collyer declined to enforce her decision, pending an appeal to a higher court. That appeal was filed in July and is still months away from resolution.

If Trump wanted to seriously damage Obamacare, he could simply order the appeal dropped—letting the lower-court ruling stand—and stop reimbursing insurers who are giving deep discounts to half their customers. And that would wreak havoc, said Michael Cannon of the libertarian Cato Institute, a longtime opponent of the health law. The insurers would still have to provide the discounts, as required by law, he said, “but they’re no longer getting subsidies from the federal government to cover the cost. So they are going to be selling insurance to these people way below the cost of that coverage.”

Even those who support the law say that would effectively shut down the exchanges, because insurers would simply drop out. A new Trump administration “really could collapse the federal exchange marketplace and the state exchanges if they end cost-sharing” payments to insurers, said Rosenbaum, who has been a strong backer of Obamacare. There is already some concern about the continuing viability of the exchanges after several large insurers, including Aetna and UnitedHealthCare, announced they wouldn’t participate in 2017.

Another way Trump could undermine the law would be by simply not enforcing its provisions, particularly the “individual mandate” that requires most people to have insurance. That requirement is supposed to ensure that both healthy and sick people sign up, thus spreading the costs of people with high bills across a larger population. But “executive branch non-enforcement could make a real difference to the vitality of the exchanges going forward,” Bagley told me. If healthy people don’t sign up, sick people would need to pay more money for their insurance.