The Affordable Care Act signup page on the HealthCare.gov website in 2013. (Mike Segar/Reuters)

THE AFFORDABLE Care Act, also known as Obamacare, has endured attack after attack, yet it has not collapsed. Instead, it proves repeatedly that it fills a substantial gap in the U.S. health-care system. This should finally cause some reflection among those who have been trying to kill it.

President Trump’s Health and Human Services Department admitted this month that 11.8 million people signed up for private insurance plans through the Obamacare marketplaces this year, despite slashed funding for advertising and an open-enrollment period that was shortened by half. HHS played up a rise in premiums relative to last year’s, but most people on the Obamacare exchanges receive federal subsidies, keeping their costs steady. The average subsidized premium is only $89 per month.

People have voted with their enrollment decisions: A sizable number of Americans do not get insurance from their employers and value the coverage on Obamacare’s markets. That refutes the GOP myth that the program forces Americans to purchase junk insurance that they do not want. A recent Kaiser Family Foundation poll found that these consumers seek to guard against major medical costs, to gain the peace of mind that comes with insurance and to obtain coverage for chronic medical care, suggesting that the law serves important and durable needs.

Another fictional Republican claim is that Obamacare has been collapsing. A Kaiser study this year found that insurance markets stabilized in 2017, despite Mr. Trump’s best efforts to undermine the law. This comports with findings from the Congressional Budget Office and a range of other independent analysts.

But not all the news is good. Enrollment is down nearly 1 million people from a 2016 peak of 12.7 million. The decline came in states where the federal government is running insurance marketplaces; enrollment in the states that run their own marketplaces held steady. Some insurers have exited the market.

Then, too, Republicans have launched a new wave of attacks, the consequences of which won’t be fully visible for months or years. Congressional Republicans eliminated Obamacare’s individual mandate, setting it to end next year. As this year’s enrollment figures suggest, many Americans who rely on the law’s protections and subsidies will continue to buy Obamacare plans, but insurance customers who feel healthy will face fewer incentives to stay in the system, even if it leaves them one car accident away from financial ruin. New HHS rules also threaten to erode the enrollment of healthy customers in comprehensive Obamacare plans by promoting cheaper, skimpy plans, which will make it harder for insurers to maintain the financial stability of the plans that cover care for sick people. Meanwhile, Congress failed to pass any of the bipartisan Obamacare stabilization bills that lawmakers had negotiated.

Obamacare continues to serve an important need. What’s sad to see is how easy it would be to make it even more useful, if Republicans would focus on improvement instead of sabotage.