After a rough incubation, birth and infancy, the Affordable Care Act is a sturdy 5-year-old, showing increasing signs of achievement and acceptance. But growth could come to a sudden halt if the Supreme Court throws out the subsidies that enable more than 8 million Americans to pay their health insurance.

That would energize opponents, who have spent six years fighting Obamacare. But the negative fallout would extend beyond President Barack Obama to millions of beneficiaries.

Recent progress makes it much harder to justify contentions the controversial law is fundamentally unsound. The number of uninsured Americans has dropped dramatically, the rate of health care cost increases is trending down and some less-publicized, longer-term benefits are starting to show up.

The overall public remains divided on the law itself. But support has risen and, among recipients, even a majority of Republicans are satisfied with their health care, a recent Reuters/Ipsos survey disclosed.

“One still sees a lot of noise in the system, but the facts and the traction of the substance of it working are beginning to take hold,” Health and Human Services Secretary Sylvia Mathews Burwell said in an interview.

The Kaiser Family Foundation’s latest monthly tracking poll showed a narrow margin of the public views the law favorably. While a slight majority in the Reuters/Ipsos poll was unfavorable, 60 percent of those covered by the law were favorable and two-thirds of them were satisfied with their health care, including 53 percent of Republicans.

Nevertheless, congressional Republicans included Obamacare repeal in their budget, while GOP presidential candidates vow its replacement. But the Supreme Court poses the principal threat to its continued growth — even its existence.

Burwell and other administration officials express confidence the court will uphold subsidies in the 34 states where state inaction forced recipients to purchase insurance through the federal government’s exchange. They feel the legislative history makes clear Congress’ intent, despite language specifying recipients as those in exchanges “established by the state.”

The nonpartisan Urban Coalition says a negative ruling could deprive 9.3 million Americans of tax credits — including 1.5 million Texans receiving $4.3 billion — and cost 8.2 million their health insurance. The federal government would be unable to respond administratively. It would detract from ongoing efforts to do what Congress mostly can’t do: make the complex law work better.

Obamacare’s impact is growing. The government says a net total of 16.4 million people have gained health insurance — despite another 6 million losing it — since ACA’s enactment. Nearly 30 percent picked up coverage from an expanded Medicaid, and almost as many were young adults who gained eligibility to stay on a parent’s plan.

Predicted long-term financial benefits are emerging. Annual health care cost growth has slowed to 3.2 percent, the lowest in four decades, reducing Congressional Budget Office long-term cost estimates. The rates of both Medicare hospital readmissions and hospital-acquired conditions have declined.

An adverse court ruling would reverse that trend and, Burwell recently wrote to lawmakers, “cause massive damage.”

But it might put even more pressure on the GOP’s congressional majority. As longtime GOP operative Karl Rove wrote in his weekly column in The Wall Street Journal, “Republicans better be ready to say what to do next,” lest they get blamed for depriving millions of their health insurance.