Parts of the law that would need to be unwound include no-charge preventive services for older Americans on Medicare, allowing parents to keep children on their plans through age 26, a variety of efforts to rein in prescription drug costs and even requirements that some restaurants post calorie counts

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“It affects almost everyone in America,” said Tim Jost, a specialist in health law and a professor emeritus at Washington and Lee University.

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The court decision also sets up an awkward juggling act for the Republican Party, balancing President Trump’s gleeful tweets over the court’s decision against a perception that the GOP is threatening insurance that ­covers nearly 20 million Americans through new private health plans and an expansion of Medicaid — a perception that Democrats successfully employed in last month’s elections that gave them control of the House.

While the ruling by U.S. District Judge Reed O’Connor invalidated the law, it did not enjoin any of its provisions. The Trump administration immediately announced that the law the president sought to dismantle would stay in place for now.

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Over the weekend, Republicans and Democrats alike predicted it could take a year or more before the true significance of Friday night’s opinion by one conservative federal judge is known. Echoing the Trump administration, they said the ruling would begin a long path through the appellate courts, probably bringing the question of the ACA’s constitutionality before the Supreme Court for a third time.

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On Sunday, Senate Minority Leader Charles E. Schumer (D-N.Y.) joined Nancy Pelosi, who is expected to be the next speaker of the House, in promising to make Democrats part of an appeal. Schumer said on NBC’s “Meet the Press” that “the first thing we’re going to do when we get back there in the Senate is . . . put a vote on the floor, urging an intervention in the case.” Unlike the House, the Senate will remain in Republican control.

Nevertheless, the ruling, with its sweeping breadth, unsettles what had at least seemed a greater calm this year for the sprawling law, which has been challenged by its own shortcomings and its Republican political nemeses ever since Democrats pushed it through Congress in 2010. O’Connor’s ruling came out of the most recent Republican challenge, brought by nearly 20 state attorneys general from red states.

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If it stands, the most politically delicate part of the law at stake is its rule forbidding insurers from charging more to people with preexisting medical conditions or refusing to cover them at all.

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“This can’t happen!” was Christine Nelson’s first thought when she saw reports of the ruling on the news in Phoenix.

Nelson has metastasized melanoma. Without the ACA, she feared, insurers could return to placing yearly and lifetime limits on the cost of her treatment, which has run up more than $1 million in the past dozen years for eight surgeries, radiation on four parts of her body, eight different immunotherapies and so many scans and biopsies that she has lost count. And she and her husband, who runs a small heating and air conditioning business and has Type II diabetes, could be frozen out of health plans without the law’s protections.

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“Without any type of government regulation or anything, I would be dead,” said Nelson, 56, a registered Republican who left her job as an economics teacher six years ago because she is too ill to work.

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The most immediate impact of the decision, coming a night before the busy final deadline in most states for people to enroll in ACA health plans for 2019, was confusion for the tens of thousands of people who typically sign up at the last moment.

That timing created a maximal shudder across the health-care industry.

Even attorneys for the Justice Department, which had refused to defend the ACA at the administration’s behest, had asked O’Connor to hold off issuing any injunction until after open enrollment.

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“The last thing we want is for there to be chaos in the insurance markets and for people who currently have health care to lose that health care,” Deputy Assistant Attorney General Brett Shumate told the judge in September.

As it turned out, O’Connor did not wait.

On Saturday, Andy Slavitt, former administrator of the Centers for Medicare and Medicaid Services (CMS) under Barack Obama, called on the Trump administration to extend open enrollment. “Typically, over 650,000 people sign up on the final day of open enrollment,” Slavitt tweeted, “so the potential for confusion today is highly impactful.”

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The CMS did not extend the deadline. Federal figures due out in a few days will show whether the latest legal peril to the law prompted consumers to shy away on what is typically the biggest enrollment day of the season — or remember to sign up as news of the ruling prompted heightened attention to the deadline.

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Over the weekend, the administration tried to stave off consumer worries. “The exchanges are still open for business and we will continue with open enrollment,” current CMS Administrator Seema Verma tweeted a few hours after the ruling. “There is no impact to current coverage or coverage in a 2019 plan.” The CMS oversees much of the ACA.

An agency spokesman said Saturday that, as in the five previous ACA sign-up periods, “we have seen an increase in the volume of consumers contacting our call center and visiting HealthCare.gov during the final days of open enrollment.”

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Some insurance seekers were asked to leave contact information with the call centers and would be allowed to finish choosing a health plan after the deadline, the spokesman said, without providing figures.

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For the insurance industry, the ruling brings new confusion just as years of political turmoil, major insurers withdrawing from marketplaces and spiking rates had finally given way to some sort of stability.

“This year we got back to some semblance of normal,” said Kristine Grow, spokeswoman for America’s Health Insurance Plans, the industry’s main trade group. Despite the ruling, “nothing is changing with the law today. We are going to continue business as usual,” she said.

Justine Handelman, senior vice president at the Blue Cross Blue Shield Association, whose members play a large role in the individual marketplaces, said, “I don’t think it will have an impact on 2019. . . . But certainly, as you do look to the future, it does bring in uncertainty.