This latest legal threat to Obamacare was filed by a group of red states in February 2018, months after Republican-led efforts to repeal the law collapsed in Congress. They argue that Congress’ decision to scrap the individual mandate penalty in its 2017 tax cut rendered the law unconstitutional because the Supreme Court previously upheld the mandate as a valid exercise of taxing power. Congress lowered the penalty for not purchasing health coverage to $0, but the mandate remains on the books.

In December, U.S. District Court Judge Reed O’Connor sided with the Republican-led states, shocking legal experts. The lawsuit was once seen as a long-shot, but it’s received serious consideration by Republican-appointed judges.

Appellate Judge Jennifer Elrod, a George W. Bush appointee, on Tuesday posited that lawmakers — who failed to agree on an Obamacare replacement plan two years ago — deliberately eliminated the mandate penalty because they knew the rest of the law would have to fall. She said perhaps lawmakers thought, “Aha, this is the silver bullet that’s going to undo Obamacare.”

Attorneys for the 20 Democratic-led states that are defending the law, as well as the Democratic-controlled House, countered that Congress clearly intended for the rest of the law to survive when it eliminated the mandate penalty.

“All the court has to do is look at the text,” said Samuel Siegel, the attorney representing the Democratic-led states.

The three-judge appellate panel is expected to rule in the coming months. They could back the lower court ruling invalidating all of Obamacare or overturn it entirely. The judges may also determine that the elimination of the individual mandate penalty only renders certain parts of the ACA unconstitutional, such as its protections for individuals with preexisting medical conditions. That was the Trump administration’s original stance on the lawsuit before recently embracing the lower court ruling against the entire ACA.

Judge Kurt Engelhardt, a Trump appointee, pointed out that Congress could settle the dispute over the health law’s future by immediately stripping out the individual mandate entirely, eliminating the basis for the lawsuit. He also questioned why the Republican-controlled Senate hasn’t weighed in on the lawsuit.

“They’re sort of the 800-pound gorilla who’s not in the room,” Engelhardt said.

But in Washington, Senate Republicans were not eager to talk about the case, and several are betting privately that the ACA will survive. Some expressed concern a court decision throwing out the entire law could create chaos in the health care system if it took effect immediately.

“If the appellate court upholds the lower court’s decision, then I hope they’re able to provide a transition,” said Sen. Kevin Cramer (R-N.D.). “If the court decides it’s not [constitutional], then we have our work cut out for us.”

Senate Majority Leader Mitch McConnell on Tuesday said Congress would act “quickly, on a bipartisan basis” to restore preexisting condition protections if the courts struck them, but there’s no guarantee that they would. Republicans have supported less robust protections than those provided by the ACA, and Democrats aren’t likely to accept anything less.

Democrats, who saw their promises to defend coverage and pre-existing conditions as winning campaign messages in 2018, are eager to again press that case in 2020.

“The stakes cannot be higher,” said Senate Minority Leader Chuck Schumer. “Millions of Americans will suffer if Republicans succeed. Shame on them.”

Still, what happens next in the courts isn’t clear. During the Tuesday hearing, theRepublican-appointed judges raised numerous questions about whether any party had any legal standing to challenge the ACA’s constitutionality in the first place or to appeal the federal judge’s earlier ruling against the ACA.

The standing issue emerged as a recent wild card in the legal proceedings — the appeals panel two weeks ago first raised questions about whether anyone could challenge the decision after the Trump administration in March shifted its legal strategy to fully side with the Republican-led states.

The judges could toss the entire lawsuit if they determined the red states who brought the case haven’t suffered any harm from the removal of the individual mandate penalty. Attorneys for those states argued that even without a tax penalty, the mandate causes harm by forcing them to spend money on government health care coverage for more people.

“The ACA causes classic pocketbook injury to the states,” said Kyle Hawkins, the attorney representing the Republican-led states.

Meanwhile, the judges seemed confused by the Trump administration’s legal position. The Department of Justice supports the lower court ruling against the entire ACA, but at the same time it has argued that some provisions of the law — which the DOJ hasn’t specified — should remain.

DOJ attorney August Flentje argued that it’s too soon to determine exactly which provisions can remain even if it’s struck down.

“A lot of this stuff will need to be sorted out,” Flentje said. “It’s complicated.”

The lone Democratic appointee on the three-judge panel was silent throughout the hearing. The judges gave no indication on when they might rule on the case.

The ACA has remained in effect since a federal judge found it unconstitutional last December, and the Trump administration has said it will enforce the law while the legal battle continues.

Alice Miranda Ollstein and Adam Cancryn contributed to this report.