The slippage has especially hurt children — a recent analysis of new census data by The Times found that the number of children without insurance rose by more than 400,000 between 2016 and 2018.

Now, as the coronavirus sweeps through the country, many state officials are relying on the Affordable Care Act to provide health coverage for residents who have none. On Friday, California became one of the latest states to set up a special enrollment period so people can sign up for insurance on their state-run marketplaces, and the Trump administration is considering reopening enrollment in the larger federal marketplace, which serves most states, for a limited period.

It hasn’t curbed costs enough.

For many Americans, the “Affordable” part of the Affordable Care Act has seemed like an empty promise, as premiums, deductibles and other out-of-pocket costs continue to be an extraordinary burden on millions of households.

But the law has made health care far more affordable in a number of less conspicuous ways.

For Marque Dailey of Dallas, 35, who has multiple sclerosis, the Affordable Care Act was the only way to get private insurance. Before the law, insurance companies were allowed to deny coverage to people like him who had expensive medical conditions, or to charge such a high price that many could not afford the premiums. About half of all Americans had such pre-existing conditions, including high blood pressure or lung disease, that resulted in their being denied or potentially priced out of coverage, according to one federal estimate.

After the law passed forcing insurers to accept anyone without raising premiums, Mr. Dailey was able to enroll in a Blue Cross plan in Texas, which covered his medical care that at times approached $200,000 a year. His income was low enough that he also qualified for generous federal subsidies under the law that kept his monthly premiums at no more than $235, and his out-of-pocket costs capped at around $1,000 a year.

The law has also played an important role in keeping care affordable for the 160 million Americans who get coverage from an employer, including by requiring those plans to cover the children of beneficiaries until age 26.

Before the law, employer-provided plans often set strict limits on what they would pay toward medical bills during a single year and over a lifetime. An estimated 105 million Americans had some sort of lifetime cap before the passage of the health care law.