WASHINGTON (AP) — The repeal of an unpopular fine for people without health insurance has had little impact on "Obamacare" sign-ups or premiums, a gap between the real world and legal arguments from conservatives again challenging the Affordable Care Act.

The 10-year-old law has proved more resilient than its creators or detractors imagined, even as the Supreme Court considers whether to take up the latest effort to roll it back.

Opponents argue that the constitutionality of the entire 900-page law hinges on the now-toothless penalty for not having health insurance. Collected as a tax by the IRS, the penalty was intended to enforce the law's "individual mandate" that Americans be insured. A previous Republican-led Congress set the fines to $0, effective last year.

"We've gotten a lot of evidence by now about what the market looks like without a mandate penalty, and on the whole it looks pretty stable, which is surprising because that's not what most people would have expected when the ACA was being written," said Cynthia Cox, who directs research on the health law for the nonpartisan Kaiser Family Foundation.

A Kaiser study released this week found that removal of the penalty pushed premiums up about 5% going into 2019, but the bottom line was a wash because of other factors. Insurers appeared to be making healthy profits.

The penalty was thought to be critical when the law was being written in 2009-2010. The idea was to nudge healthy people to sign up, helping keep premiums in check. But Cox said there's no indication that healthy people have dropped out in droves. In one telling statistic, the Kaiser study found that average hospital days per 1,000 people enrolled dipped slightly in 2019, even after the penalty was eliminated.

Preliminary government enrollment numbers for 2020 point to stability, with about 8 million people signed up in the 38 states served by the federal HealthCare.gov website. A final count including states that run their own sign-up efforts is expected by the spring.

The insurance mandate was the central issue when the Supreme Court first upheld the health care law in 2012, over a year before HealthCare.gov opened for business.

Chief Justice John Roberts cast the key vote in that 5-4 decision. He found that Congress lacked constitutional authority to require that Americans have health insurance. But because Congress has broad powers to levy taxes, Roberts ruled that a tax on people who did not purchase coverage offered them was constitutional. That allowed the law to survive what's still seen as its most serious legal challenge.

Kathleen Sebelius, health secretary for President Barack Obama, said in 2012 that it was generally accepted that the insurance mandate was part of a three-legged stool key to stable markets. The other two legs were taxpayer-provided subsidies for premiums and a guarantee that patients with preexisting medical conditions could no longer be turned down or charged more.

"It was thought that the trade-off for changing the rules on preexisting conditions would have to be ... some penalty incentive so you would get healthy people in the pool, along with not-healthy people," Sebelius said. "What became clear when the law went into effect (in 2014) is that the subsidies in many ways provided a greater incentive for people get health insurance."

Those subsides are designed so that low- and moderate-income households only spend a fixed percentage of their incomes on premiums, shielding consumers from high sticker prices.

Cox agreed that the law's "carrots" seem to have made more of a difference than its "stick."

Fast-forward to 2018 and a coalition of conservative states led by Texas won a lower court decision that the insurance mandate was still critical, in a legal and constitutional sense.

U.S. District Court Judge Reed O'Connor in Texas ruled that by zeroing out the tax penalty, Congress rendered the insurance mandate unconstitutional, and without it the entire health law must fall. President Donald Trump agreed.

Recently, a federal appeals court in New Orleans agreed with O'Connor that an unenforceable insurance mandate is unconstitutional. But the appeals court sent the case back to him to see whether other parts of the law can stand.

Defending the law, a coalition of Democratic-led states, along with the U.S. House, appealed to the Supreme Court, seeking a fast-track decision amid this year's presidential election. The court has asked lawyers for the conservative states to respond by Friday on the timing question.

University of Michigan law professor Nicholas Bagley said the stability of the health insurance markets exposes "the artificiality" of the conservatives' argument.

"It really goes to show how ridiculous it is to claim that Congress understood the mandate to be so essential that if it were to be red-lined out, the rest of the law would have to fall," said Bagley.

Not so fast, said Andrew Schlafly, a lawyer representing groups siding with Texas and the other GOP-led states opposing the law.

"The question is not whether in reality (the ACA) can work without the mandate," said Schlafly. "The test is whether it was intended to work without the mandate.

"Theory does matter to these Supreme Court justices," he added, "and they do take theory seriously."