It is possible that regulators could devise a new way to calculate the subsidies. But in my conversations with several experts on the law, no one was quite sure how. When Congress considered repealing the Affordable Care Act last year, most of the plans changed rather than eliminated those subsidies. Taking them away altogether would mean millions fewer people would have health insurance, according to estimates from the Congressional Budget Office.

(Even if regulators found a way to preserve the subsidies, the subsidies would be useful only for Americans healthy enough to qualify for insurance. For the sick, “it’s a Groupon that’s expired,” said Mark Hall, the director of the health law and policy program at Wake Forest University.)

Community rating is also important for Obamacare’s online insurance marketplaces. Now, people who want health insurance can go to one website and shop for plans, comparing benefits, deductibles and prices. They can choose a plan and sign up within hours.

But with no guaranteed coverage and no standard premiums, it would be impossible to know whether you could buy any particular plan, or at what price. Customers would need to apply to each plan separately, filling out detailed medical histories — and in some cases submitting blood and urine samples — to find out what they would be allowed to buy. Those steps would make shopping much more difficult. Obamacare currently limits the period during the year when people are allowed to shop for insurance, meaning that customers might have to submit multiple applications at once in order to be able to buy insurance before the shopping period closed.

There’s also reason to think that, in a world where community rating and guaranteed issue disappear but the rest of Obamacare remains, many more people would be turned down for insurance than in the pre-A.C.A. days. That’s because the Trump administration wants to leave other insurance regulations in place that didn’t exist before the health law. Obamacare says that all health plans need to cover a standard set of health benefits and can’t limit the amount they will pay in a year. In the old days, plans often didn’t cover prescription drugs or treatment for mental illness. Those strategies helped weed out customers who had expensive health needs and limited the companies’ risk.

In a system where the plans would need to cover expensive drugs for H.I.V. or inpatient treatment for schizophrenia, they might be more leery of taking on any customers at risk of needing such services. “I would think that insurers would end up being stricter in their exclusions of people because they couldn’t limit benefits,” said Larry Levitt, a senior vice president at the Kaiser Family Foundation, a health research group.

There is no guarantee that the lawsuit will turn out the way the Trump administration wants, of course. And any final answer is probably a long way away, since the case is likely to be appealed once it is decided. A group of Republican states that brought the lawsuit initially argued that the entirety of Obamacare should be invalidated. A decision for those litigants would mean changes not just to the individual market but to Medicare, Medicaid and a broad array of other health programs.

But if the court rules in the way the Justice Department has requested, the result will be a world in which it is much more difficult for individuals and small businesses to buy health insurance. The health law is complicated, with interlocking parts. It will be hard to remove just two rules from the individual market without disrupting many others.