If it does end up before the Supreme Court, the fate of the ruling will probably hinge on Chief Justice John G. Roberts Jr., the conservative who in the two previous cases wrote the opinions that dismissed the challenges.

And this time, it would come without the galvanized support of the conservative legal establishment that has marked previous lawsuits and in a political climate in which even Republican lawmakers are more likely to talk about replacing Obamacare than destroying it.

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Still, legal experts cautioned against dismissing U.S. District Judge Reed O’Connor’s decision as an implausible flight of judicial activism that is sure to be quickly struck down, as analysts have done with past ACA challenges. The cases are usually taken far more seriously by the judges who review them than the lawyers who analyze them, and they have resulted in close votes at the Supreme Court.

“People should not under­estimate this challenge,” said Josh Blackman, a law professor at South Texas College of Law in Houston who has written two books about the previous legal efforts to derail the ACA. “It is not a frivolous case.”

O’Connor, based in Fort Worth, ruled Friday night that a decision by the Republican-led Congress in 2017 to eliminate the tax penalty for not complying with the ACA’s individual mandate knocked down its legal underpinnings. The entire act must fall, O’Connor wrote.

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On Monday night, California Attorney General Xavier Becerra and 16 Democratic counterparts who opposed the lawuit filed a legal brief asking the judge to clarify that he intends for the ACA to remain in effect until appeals are completed. If O’Connor does not have that intention, the motion asks that he grant a stay.

“The ACA touches nearly every aspect of the nation’s healthcare system,” the Democratic attorneys general wrote. “Treating the court’s order as immediately effective would create widespread harm and confusion.”

They asked O’Connor to rule on their request by Friday and to take the necessary legal steps so that they can appeal it.

Blackman acknowledged that other members of the conservative legal establishment who have fought previous battles against the ACA are not as convinced as he is that O’Connor’s decision is, at least in part, defensible.

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The conservative Wall Street Journal editorial board, after declaring that “no one opposes Obamacare more than we do,” blasted O’Connor’s ruling both on the basis of the law and politics.

“Millions of people now rely on ObamaCare’s subsidies and rules, which argues against judges repealing the law by fiat,” the board said Monday in an editorial. “Judge O’Connor breezes past this like a liberal Ninth Circuit appeals judge handling a Donald Trump appeal.” The reference is to the U.S. Court of Appeals for the 9th Circuit, which Trump has accused of having partisan judges who oppose his initiatives.

Jonathan H. Adler and Abbe R. Gluck, law professors who were on opposite sides during the 2012 and 2015 challenges to the law, together denounced Friday’s ruling.

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It is “an exercise of raw judicial power, unmoored from the relevant doctrines concerning when judges may strike down a whole law because of a single alleged legal infirmity buried within,” they wrote in a New York Times op-ed.

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And while Trump has praised the ruling, his administration has been quick to note that O’Connor did not issue an injunction against the ACA and that it remains in effect.

The Department of Health and Human Services issued a statement Monday declaring that it “will continue administering and enforcing all aspects of the ACA as it had before the court issued its decision.”

O’Connor’s decision came because of a suit filed by 20 Republican state attorneys general, but there has been little public rejoicing among congressional Republicans.

“Obamacare burnout” is how Blackman characterized the reaction. The public has come to take for granted some of the ACA’s most popular features — guaranteeing insurance for those with preexisting conditions, for instance, and allowing children to remain on their parents’ policy until age 26 — and the GOP is loath to take responsibility for ending them, he noted.

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O’Connor’s decision has its origins in the Supreme Court’s 2012 ruling on the law. Roberts joined the court’s conservatives in saying that in giving Congress power to regulate interstate commerce, the commerce clause of the Constitution did not give it the authority to require Americans to purchase health insurance.

But then Roberts joined with the court’s liberals to save the law, writing that the penalty imposed for not complying with the individual mandate was a legitimate exercise of authority because Congress “does have the power to impose a tax on those without health insurance.”

After Trump won the White House and Republicans seized control of Congress, there was another effort to repeal the ACA. It fell short. Instead, as part of a tax overhaul, congressional Republicans pushed through a change in which the ACA penalty will be eliminated, starting in January.

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The lawsuit by the Republican attorneys general argued that, with the enforcement of the insurance requirement gone, there is no longer a tax — so the law no longer is constitutional.

In his 55-page opinion, O’Connor agreed. He wrote that the individual mandate is unconstitutional, saying that it “can no longer be fairly read as an exercise of Congress’ tax power.”

The Justice Department, which in the past had defended the ACA, had asked O’Connor for just such a ruling.

But O’Connor went further, ruling that the entire law must fall because the individual mandate was so essential.

That position has prompted the most criticism. Courts are supposed to look for ways to salvage federal laws by excising only the parts that are unconstitutional. The theory is that Congress would rather have part of the law survive than none of it.

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Critics of O’Connor’s ruling said he ignored the fact that Congress clearly thought in 2017 that it could remove the penalty for not complying with the individual mandate while keeping other provisions in the law intact.

“Judge O’Connor’s analysis of the severability issue is badly flawed,” wrote Ilya Somin, a professor at George Mason University’s Antonin Scalia Law School who had argued that the original ACA was unconstitutional. He added, in a post on the website Volokh Conspiracy, “It just doesn’t make any sense to conclude that an essentially toothless mandate is ‘essential’ to the ACA.”

Many say they do not expect O’Connor’s ruling to survive review at its next stop, the U.S. Court of Appeals for the 5th Circuit, even though it is viewed as one of the nation’s most conservative appeals courts.

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Somin is not sure. “The fact that one federal judge has endorsed the states’ severability argument increases the odds that others might, as well,” he wrote. “The history of ACA-related litigation is filled with surprises and failed predictions by experts.”

If the 5th Circuit overturns O’Connor’s ruling, the controversy could end there. If not, the Supreme Court would almost surely accept an appeal for the term that begins in October 2019, with a decision coming in the presidential election year of 2020.

There are no planned departures among the justices who upheld the act twice before: Roberts, Ruth Bader Ginsburg, Stephen G. Breyer, Sonia Sotomayor and Elena Kagan. Democrats and liberal groups opposed Trump’s choices for the court, Neil M. Gorsuch and Brett M. Kavanaugh, by saying the fate of the act might be in their hands. But both replaced justices who in 2012 would have found the entire law unconstitutional.

O’Connor drew extensively on Roberts’ 2012 opinion that said the law could not be justified by the commerce clause, and that Congress’s taxing power provided its legitimacy.

It is possible the chief justice would be moved by that argument and conclude that the individual mandate, and along with it the provisions that provide affordable insurance for those with preexisting conditions, should fall.

But finding that the entire law is unconstitutional might be a more difficult ask. Roberts in 2012 quoted Justice Oliver Wendell Holmes when explaining that “as between two possible interpretations of a statute, by one of which it would be unconstitutional and by the other valid, our plain duty is to adopt that which will save the act.”

It is also unlikely that Roberts would relish a third go-round on the law.

In the 2012 opinion, the chief justice went out of his way to portray the court as adjudicating matters of law, not public policy.

“Those decisions are entrusted to our nation’s elected leaders, who can be thrown out of office if the people disagree with them,” Roberts said. “It is not our job to protect the people from the consequences of their political choices.”