Most Americans would still be able to get coverage under a plan provided by an employer or under a federal program, as they did before the law was passed, but protections for pre-existing conditions are particularly important to those who want to start their own businesses or retire early. Employers would sometimes refuse to cover certain conditions, and companies would have to decide if they would drop any of the conditions they are now required to cover.

The need to protect people with existing medical conditions from discrimination by insurers was a central theme in the midterm elections, and Democrats attributed much of their success in reclaiming control of the House of Representatives to voters’ desire to safeguard those protections. Many Republicans also promised to keep this provision of the law, although exactly how was unclear. Before the law, some individuals were sent to high-risk pools operated by states, but even that coverage was often inadequate.

171 MILLION

Americans who no longer face caps on expensive treatments.

The 156 million Americans who get coverage through an employer, as well as the roughly 15 million enrolled in Obamacare and other plans in the individual insurance market, are protected from caps that insurers and employers used to limit how much they had to pay out in coverage each year or over a lifetime. Before the A.C.A., people with conditions like cancer or hemophilia that were very expensive to treat often faced enormous out-of-pocket costs once their medical bills reached these caps.

While not all health coverage was capped, most companies had some sort of limit in place in 2009. A 2017 Brookings analysis estimated that 109 million people would face lifetime limits on their coverage without the health law, with some companies saying they would cover no more than $1 million in medical bills per employee. The vast majority of people never hit those limits, but some who did were forced into bankruptcy or went without treatment.

60 MILLION

Medicare beneficiaries would face changes to medical care and possibly higher premiums.

About 60 million people are covered under Medicare, the federal insurance program that covers people over 65 years old and people with disabilities. Even though the main aim of the A.C.A. was to overhaul the health insurance markets, the law “touches virtually every part of Medicare,” said Tricia Neuman, a senior vice president for the Kaiser Family Foundation, which did an analysis of the law’s repeal. Overturning the law would be “very disruptive,” she said.

Medicare beneficiaries would have to pay more for preventive care, like a wellness visit or diabetes check, which are now free. They would also have to pay more toward their prescription drugs. About five million people faced the so-called Medicare doughnut hole, or coverage gap, in 2016, which the A.C.A. sought to eliminate. If the law were overturned, that coverage gap would widen again.

The law also made other changes, like cutting the amount the federal government paid hospitals and other providers as well as private Medicare Advantage plans. Undoing the cuts could increase the program’s overall costs by hundreds of millions of dollars, according to Ms. Neuman. Premiums for as many as 55 million people under the program could go up as a result.