When the Supreme Court hears a case Tuesday that could abolish the Affordable Care Act, the stakes will be higher than ever, coming amid a historic health and economic crisis that has deprived millions of Americans of insurance and cast a neon light on health care’s importance.

A decision this term to strike down the entire ACA — unlike when justices upheld the law on different grounds in 2012 and 2015 — would upend the health-care system in ways that touch most people in the United States.

If the high court, with a newly strengthened conservative majority, were to overturn the law following this week’s oral arguments, health insurance would collapse for at least 23 million Americans. Nearly 11 million would lose coverage through marketplaces created under the ACA to sell private health plans, usually with federal subsidies, to people who cannot get affordable benefits through a job. And about 12 million, insured because of Medicaid expansions in all but a dozen states, could find that coverage disappear.

According to health policy specialists, the consequences would ripple significantly beyond those losing their insurance outright. Flu shots, cancer screenings and other preventive care — including the cost of administering a coronavirus vaccine once it exists — would no longer be free of charge. Older customers would pay higher insurance rates. Young adults no longer could stay on their parents’ insurance policies through their mid-20s. And most health plans no longer would be required to cover as many types of care.

“Overturning the ACA would affect almost everyone, one way or another,” said Larry Levitt, senior vice president of the Kaiser Family Foundation, a nonpartisan health policy group. It would “put the health system into chaos and the political system into chaos, as well,” he said.

Georges C. Benjamin, executive director of the American Public Health Association, noted that the law’s end would take away money from community health centers, the Centers for Disease Control and Prevention and already strained rural hospitals.

Such far-flung effects would materialize no matter when the high court decided the law was unconstitutional.

The 2,000-page law adopted in 2010 by a Democratic Congress was a main domestic achievement of the Obama presidency. In the decade since, the statute’s many moving parts have become entrenched in the habits and expectations of insurance companies, hospitals, doctors’ offices and state governments — not to mention consumers of medical care.

But the renewed possibility that the high court could find the sprawling law unconstitutional carries particular intensity at this moment, months into the coronavirus pandemic, which has sickened more than 9.8 million people in the United States and caused more than 237,000 updated deaths.

For those who have been infected and are aware of the court case, one of the greatest fears involves a part of the law that consistently has ranked as the most popular in years of public opinion polling: protections for people with preexisting medical conditions. The protections forbid insurers to charge such customers higher prices, to refuse to cover them for care associated with their condition, or to refuse to sell them a health plan at all.

Covid-19, the illness caused by the coronavirus, could be classified as a preexisting condition, throwing into doubt coverage of ongoing treatment for some patients who suffer lingering effects and sometimes need care long after they contract the virus.

“It just feels so unfair to take away anything that can help me right now, when there are so few things that can help me right now,” Mary Cameron said of the possibility she could eventually be denied insurance.

Cameron, 32, was an ultramarathon runner for a decade and earned a PhD from Stanford University last year focused on renewable energy and atmospheric modeling. She began working for a Silicon Valley start-up, and, when the virus arrived on the West Coast, she was the one who brought hand sanitizer for her office, stopped using transit and donated the supply of N95 masks she’d stocked for fire season to a friend who works at Stanford’s hospital.

She hadn’t been outside her apartment in three weeks when, out of produce, she made a trip to a grocery store March 31, before mask-wearing was widely recommended beyond those with symptoms.

After days in bed — her heart beating wildly, her mind foggy, sometimes feeling on the edge of passing out — she went to Stanford’s emergency room. Her fever was just below 100, the threshold for being admitted. For the visit of less than four hours, the bill for tests and the doctors she saw was $14,706 — all but $1,370 paid by her Blue Shield of California insurance.

She thought she had recovered, but then symptoms returned in August — migraines, fatigue, more mental fog. On the advice of a call-in nurse, she went back to the emergency department to rule out lung embolisms, and her insurance paid for almost all of that visit, too. Still sick, she moved in September to Phoenix, where her parents and two sisters live, taking a five-month rental so her family could help her cope. She is waiting to see a neurologist virtually but couldn’t get an appointment before January.

After taking sick leave, Cameron works remotely for now. She worries whether she will get better, whether she will have the stamina to go to an office whenever hers reopens, whether she will need to leave her job because she cannot think well, losing her health plan in the process — and perhaps unable to get another.

“That’s the scariest part; it’s been messing with my brain,” Cameron said, “and not only is that my livelihood, but it’s also my access to insurance, which means it’s my gateway to healing my brain.”

The court’s likelihood of striking down the law this time, ending insurance protections for people with covid-19 or other medical conditions, is a matter of debate. Since early 2018, when a group of Republican attorneys general launched the lawsuit that has evolved into the Supreme Court case, health law scholars who support and oppose the ACA have uniformly contended that the suit’s legal arguments are weak.

Nevertheless, a conservative federal district judge in Texas, Reed O’Connor, struck down the law in its entirety in December 2018. A year later, a panel of the New Orleans-based U.S. Court of Appeals for the 5th Circuit agreed that the law’s requirement that most Americans carry health insurance is unconstitutional, but sent back to the lower court the question of whether the rest of the law can remain without that mandate.

In the lower courts and in briefs filed with the Supreme Court, the GOP attorneys general focus on a 2017 change in tax law adopted by a Republican Congress that lowered to zero a penalty the ACA had levied on people who ignore the insurance requirement. Without that tax penalty, the argument goes, the law no longer contains the basis for the Supreme Court’s 2012 ruling that it was constitutional.

For their part, a coalition of Democratic attorneys general has argued that the penalty still exists in the law, regardless of whether it has a dollar amount. Even if the penalty were gone, they argue, the rest of the law could be “severed” — in other words, separated so that the rest of the statute remains.

President Trump opposes the law, and the Justice Department is siding with the Republican attorneys general in the case. The House of Representatives, in Democratic control since the issue of the ACA helped the party win back a majority there two years ago, is aligned with the Democratic attorneys general in favor of preserving the law.

If the Supreme Court decided merely that the insurance requirement is no longer valid, the practical effect would be slim, because no one is being fined anymore. If the justices struck down the whole law, the effects would far transcend steps Trump and his top health advisers have taken to weaken the law through executive moves.

They have abbreviated the sign-up period to buy health plans through ACA marketplaces — an annual six-week period that began Nov. 1. They have slashed federal money for advertising and other outreach activities to encourage and help people to sign up for ACA coverage. And they have rewritten rules to make it easier for customers to buy skimpy and inexpensive health plans that bypass most of the ACA’s insurance protections.

One rule change allows individuals to buy “association health plans” originally meant for businesses that band together for insurance. Another lengthened to nearly a year the duration of short-term health plans originally meant mainly as a brief bridge between jobs. An estimated 3 million people were insured last year through such short-term plans, according to a July report by the House Energy and Commerce Committee.

While those skimpy health plans do not have to meet ACA insurance rules for coverage and consumer protections, the law overall “set a floor for what constituted health insurance in this country, and that would go away,” said Dan Mendelson, founder of Avalere, a health-care consulting firm.

If the court struck down the full law, Congress would face intense pressure to restore at least parts of it — especially now, said James C. Capretta, a resident fellow at the American Enterprise Institute who focuses on health care.

“In the middle of a crisis, when the only thing people can see is you have this terrible health threat, and they have higher job-security concerns, the idea the legal system might invalidate your health benefits . . . it’s a little too much for the system to handle,” said Capretta, who predicts the court will not invalidate the law.

If the justices did, Capretta said, states would lose the money the ACA provides to pay 90 percent of the costs of covering people in expanded versions of Medicaid — just as the pandemic has devastated state revenue. He said states would lean hard on Congress for help.

Still, people whose bills for the coronavirus are being paid by Medicaid are fearful.

At the end of a European music tour in early March, Jason Snell saw a friend who had a fever in Berlin and was sick himself when he flew to New York. After leaving a well-paid corporate job five years ago for the music business, he’d bought a cheap insurance policy with a high deductible and, figuring his income would go back up, kept paying for it even after he checked the ACA website in his home state of Iowa and learned his earnings were so low that he qualified for Medicaid under the law.

Sick and without gigs during the pandemic, he moved back at age 44 to his hometown of Cedar Rapids in the spring, quarantining for a month at a hotel paid for by his parents and then moving in with them. By the time he was back in Iowa and his cough was subsiding, his heart felt “like it was flopping around like a fish,” he said, and fatigue set in. A pharmacist there, filling his prescriptions for steroids and antibiotics for covid-19-related bronchitis, found Snell in a Medicaid database. It has been a lifeline, because his inexpensive insurance policy has balked at every charge.

When symptoms returned in late summer, he had an electrocardiogram at St. Luke’s Hospital; Medicaid picked up the $16,000 bill.

“You know, I have felt so taken care of by the Medicaid,” Snell said. It hadn’t crossed his mind that it might go away until last month’s confirmation of the court’s newest justice, Amy Coney Barrett, who has criticized the court’s reasoning in upholding the law in the past.

With his income down to nothing, Snell said, “I was thinking — the idea of the ACA being flipped . . . I’d be on my own.”