Life was bad before the Affordable Care Act. Don't send us back.

The Affordable Care Act is once again under threat because Republican attorneys general have banded together to argue before the Supreme Court that the individual mandate is unconstitutional and therefore the entire law should be eliminated. While Justices John Roberts and Brett Kavanaugh seemed to signal that the high court was unimpressed by this argument, the ACA remains vulnerable until the court decides the case next year — or until the next lawsuit emerges.

As a primary-care physician, my memories of providing health care before the ACA are dominated by tragedy.

In 2007, I was a medical student and running a free clinic for the uninsured in Bedford-Stuyvesant called the Brooklyn Free Clinic. I remember so many horrifying cases: the 62-year-old man who walked in with a massive tumor protruding from his abdomen that he had clearly known about for months. The 48-year-old woman with vaginal bleeding from a simple-to-treat uterine fibroid that had been neglected for so long she needed emergency hospitalization for a blood transfusion. The hundreds of recent college graduates, artists and writers who could not afford routine preventive care because they didn't have insurance.

When I wasn't seeing patients, I was calling surgeons, cancer doctors and hospital administrators to try to convince them to provide free care for our patients, many of whom would have gone bankrupt if they had been billed for the care they received. Supporting these patients wasn't easy work, and it was certainly not sustainable. We often failed, and the patients we had been trying to help either disappeared from care again or ended up facing massive bills from lifesaving treatments. I lived in perennial disbelief that this was health care in America, and it left me with a hunger to change the system.

One of the most prominent reasons our patients did not have health insurance was because of preexisting health conditions. If they did not work for a large employer and sought to purchase a health insurance plan on their own, insurers would readily deny the application, charge an exorbitant premium or exclude the preexisting condition from the policy. The prospect of returning to life without protection from preexisting conditions haunts me for personal reasons as well. I had a history of depression as a young adult, and during a period when I was unemployed after college, that preexisting condition made it completely unaffordable for me to carry health insurance. I remember the process of calling insurer after insurer only to discover that either my health or my finances would be compromised because I didn't have a job but did have a medical history. And many individual plans did not even offer coverage for mental health.

These memories have faded because of the Affordable Care Act. The number of people who don't have health insurance has been cut nearly in half, representing 20 million more Americans who can now see a doctor when they have a medical problem. Today, 135 million Americans with similar preexisting conditions are protected from discrimination as they seek to obtain health insurance because of the ACA. If you look at the Census Bureau's report illustrating the reduction in the rate of uninsured people state by state, what you see is a picture of dramatic change. In Massachusetts, where I practice medicine, we have near-universal health coverage. Even in states such as Texas, where there has been outright hostility toward the implementation of the ACA and the uninsured rate remains the highest in the nation, 1.5 million more people have health insurance today compared with 2010.

Numerous high-quality studies have demonstrated that giving people health insurance saves lives. In particular, expanding Medicaid eligibility — one of the key tenets of the ACA — resulted in a relative reduction in death from all causes of 6% in one study, or one life saved per 239 to 316 adults gaining insurance. Another study found that Medicaid expansion via the ACA saved the lives of at least 19,200 adults ages 55 to 64 between 2014 and 2017. Unfortunately, despite these findings, 12 states still have not taken this important step to provide health care to millions of their residents, and it cost lives: If all states had chosen to expand Medicaid eligibility after the ACA, 15,600 more deaths could have been avoided between 2014 and 2017.

There is one more aspect of the Affordable Care Act that has gotten far too little attention by politicians and the media: innovation in health-care delivery. The ACA incentivized health systems to deliver better care at the same or lower cost by funding new models of care. Health care began moving toward a world in which health-care providers are paid for the value of the care they provide rather than the volume.

At my hospital, many programs were borne out of the passage of the ACA. For example, the top 5% of our most complex patients are assigned a nurse case manager or licensed clinical social worker to help coordinate their care and keep them out of the emergency room. We also built advanced big-data tools and hired a dedicated team of population health managers to track patients with high blood pressure and diabetes, or those who need crucial preventive care such as colon and cervical cancer screening. If you are a patient with diabetes and you fall out of touch with your doctor or stop getting your routine bloodwork, our tools will signal my team to reach out to you and reconnect you to care. Additionally, we now routinely screen our patients to understand whether they have poor access to food, housing, or trouble paying their bills and have a team of navigators to offer advice and assistance to help overcome those crucial social barriers to health.

The ACA is not without its faults, of course, and it has fallen short even in areas where it made things better. It has not reduced health-care costs in a way that the average American can palpably feel. In fact, far too many people are now underinsured, which means that they may have trouble paying medical bills despite having health insurance. Further, there are still 29 million people who need health insurance across the nation, and the number of people without health insurance has been rising again over the past three years. As we emerge from this pandemic, millions more will join their ranks. Unfortunately, that could be you, your children or grandchildren.

Our loved ones deserve better, and the solution already exists: Congress should make purchasing insurance on the individual marketplace more affordable by providing more generous subsidies, while also allowing anyone who needs health insurance during this pandemic the opportunity to purchase it via a special enrollment period. And all states can expand Medicaid access. Most importantly, we need a government-run public insurance option available to those Americans who want it, to create more competition in the health insurance marketplace and drive costs down.

There is no plan to replace the ACA if it is struck down. Citizen efforts to push back can make a difference: Maine and Wisconsin have already withdrawn their support for the lawsuit, and with enough pressure on governors and attorneys general, other states will follow. Elected officials have the tools to make it easier for you to get health insurance when you need it. If enough people raise their voices now, we as a nation can accept that returning to the grim pre-ACA world that still haunts me is no longer an option. Instead, we can build upon the strong foundation of the Affordable Care Act to ensure that all Americans receive excellent health care when they need it, irrespective of their job or prior health history.

Horn is a primary care physician and director of population health for the Division of General Internal Medicine at Massachusetts General Hospital, an instructor at Harvard Medical School and a public voices fellow with The OpEd Project. This piece was written for The Washington Post.