Five-year-old Jumaane Cook, right, holds a sign alongside supporters of the Affordable Care Act during a rally outside the Supreme Court prior to the high court’s hearing a second major challenge to the act in Washington on March 4. (Jim Lo Scalzo/EPA)

The Supreme Court’s decision in King v. Burwell is an enormous victory for the Obama administration and the Affordable Care Act. By upholding the provision of subsidies in the 34 states that rely on the federal insurance exchange, the court ensured that 6.4 million Americans would not lose access to affordable coverage. Instead of dealing a devastating blow to Americans with ACA insurance plans, the court handed Obamacare opponents a crushing defeat. The last serious legal challenge to Obamacare is over.

Republicans have had three major opportunities to overturn the ACA — the 2012 Supreme Court case on the individual mandate, the 2012 presidential election, and King v. Burwell. They have lost them all. Meanwhile, more than 10 million Americans have gained coverage through the ACA’s health insurance exchanges, and millions more have coverage because off the law’s Medicaid expansion.

Yet the ACA in 2015 is not where health-reform supporters envisioned it would be in 2010. Obamacare is a limited law, full of compromises that were necessary to secure its enactment. Back in 2010, liberals reassured themselves that the law’s shortcomings — the absence of a public insurance option, inadequate subsidies for people whose incomes were not very low, an insurance coverage expansion far short of universal health care — could be redressed over time. Once the ACA took root, advocates would turn to the task of improving the law. As former senator Tom Harkin (D-Iowa) said in 2010, the ACA was a “starter home …[with] plenty of room for additions and improvements.”

Instead, Democrats have spent the past five years fending off GOP attacks on Obamacare in Congress, in the courts and in the states. They have been forced into waging a defensive struggle despite Obamacare’s impressive record. It is of course too early to draw firm conclusions about all of the ACA’s effects. But the law already has considerable achievements to its credit, including a substantial reduction in the size of the uninsured population. It also has defied opponents’ dire predictions — the health insurance exchanges have been stable after a rocky start, the ACA has not wrecked the U.S. economy or hurt employment, the costs of the ACA’s insurance expansion have been far lower than expected, and medical care costs have grown at rates lower than the historical average. ACA measures have produced significant Medicare savings, though exactly how much credit the ACA should receive for the continued slowdown in national health care spending (which started before the law’s enactment) remains impossible to determine.

Moreover, President Obama’s 2012 reelection ensured that the ACA would have strong backing in the White House. And yet health-reform supporters have been fighting just to keep the ACA’s “starter home” foundation intact. In part, that is because Obamacare proved more vulnerable to legal challenges than reformers anticipated. Even though the law has survived two Supreme Court challenges, the cases contributed to the uncertainty surrounding the law and prolonged controversy over its legitimacy. The court’s 2012 decision effectively making Medicaid expansion a state option blew a major hole in the ACA’s coverage expansion strategy, leaving millions of low-income Americans uninsured.

The sustained ACA debate also reflects an era of extraordinary partisan polarization in which it may be harder for new programs to secure political legitimacy. When laws pass on party-line votes, as the ACA did, and lawmakers have an incentive to keep ideological fights alive to stave off primary challenges, enactment victories can become less authoritative and final. The battle can always be rejoined another day.

Republican resistance to Obamacare at the state level, both in terms of refusing to establish health insurance exchanges and rejecting Medicaid expansion, has been much stronger than Democrats expected. Americans have not embraced the ACA the way proponents hoped, and public opinion on the law remains sharply divided along partisan lines.

Now that the ACA has triumphed again in the Supreme Court, health reform advocates can turn to the formidable task of strengthening the law. Twenty-one states still have not expanded Medicaid. The ACA cannot realize its promise of ensuring affordable care as long as so many states — including those with large uninsured populations like Florida and Texas — are holdouts. To be sure, a growing list of GOP-led states has embraced Medicaid expansion. Over time, political pressure on state governments from a hospital industry that very much wants fewer uninsured patients will intensify. And perhaps with King settled, more Republicans will drop their opposition to Medicaid expansion.

Another task is enrolling more eligible individuals into subsidized coverage in the health insurance exchanges. As impressive as the ACA’s coverage gains have been to date, there are still more than 30 million people in the United States without health coverage. Having presumably enrolled many of the people who most wanted coverage, signing up less motivated consumers remains a challenge (though the growing tax penalty for not obtaining insurance will add motivation in coming years).

Then there is the issue of affordability. Insurance plans in the exchanges offer limited, high-deductible coverage that leaves many Americans exposed to rising medical costs. Underinsurance could replace uninsurance as a problem unless reformers succeed in improving the comprehensiveness of health plans.

Ensuring that the underlying rate of medical cost growth stays under control — which will in turn help make coverage affordable — should be a priority. Fixing the so-called “family glitch,” which has left many families without subsidized coverage, would be another important step.

Back in 2010, many progressives saw a Medicare-like “public option” insurance plan that would compete with private insurers as central to reform, only to see it fail to clear the Senate. Such a plan has no chance of passing Congress anytime soon. But states could experiment with adding a public option to their marketplaces through state innovation waivers that begin in 2017. If successful, that model could spread to other states. However, political science research on the mechanisms of “policy diffusion” suggests that political entrepreneurs will need to overcome the tendency of policymakers to ignore lessons about the effects of other governments’ policies that do not fit their worldview.

Finally, perhaps the most difficult long-term challenge is to build bipartisan support for the ACA. If the partisan war over Obamacare is to end, Republicans will have to accept that it is here to stay and focus on reforming rather than repealing it. Just as President Dwight Eisenhower gave his stamp of approval to Social Security, so too could a future GOP president grant the ACA bipartisan legitimacy. That seems like a remote prospect at the moment. But at some point, the GOP’s rhetoric will have to come to terms with Obamacare as reality.

The fight to realize the ACA’s aspirations is not over. The 2016 elections loom, and just as there are opportunities for Democrats to strengthen the law, a Republican president and Congress could take steps to weaken it in 2017. In the meantime, Democrats’ defensive struggle to protect the ACA continues. More budget battles could lie ahead during the Obama’s administration’s final 18 months, as Republicans continue to target the ACA. Republican opposition to federal agencies that are producing information on evidence-based medicine puts them at risk of being undermined. As political scientist Jacob Hacker argues, the GOP could also target the redistributive-financing mechanisms that pay for the ACA’s expansion of coverage to low-income Americans.

While more and more Americans are receiving benefits from the ACA, the law’s inherited design remains an obstacle to constituency building and the generation of self-reinforcing “policy feedback.” Unlike Social Security, Obamacare does not have a well-defined population of beneficiaries, and its benefits are diffuse. The ACA is not so much a program as a series of programs, regulations, and mandates. It treats different groups of Americans in different ways at different times, which complicates efforts to mobilize public support.

Legal scholar William Eskridge, Jr. and political scientist John Ferejohn coined the phrase “super-statute” to describe laws that establish a new normative or institutional framework, “stick” in the culture and transform the course of policymaking going forward. In his King v. Burwell dissent, Justice Antonin Scalia touched upon this idea, writing “Perhaps the Patient Protection and Affordable Care Act will attain the enduring status of the Social Security Act or the Taft-Hartley Act; perhaps not.”

The ACA may one day become a super-statute—but it is not quite there yet.

Jonathan Oberlander is professor and Vice Chair of Social Medicine and Health Policy & Management at the University of North Carolina-Chapel Hill.

Eric Patashnik is Professor of Public Policy and Politics and Director of the Center for Health Policy at the University of Virginia.