The panel decision largely affirmed a ruling in December 2018 by U.S. District Judge Reed O'Connor in Texas. But the appellate panel sent back to O'Connor the question of whether Congress intended other provisions of the law to remain operable. O'Connor had earlier decided that the mandate was so central to the law that the entire law must fall.

If the entire Affordable Care Act is unconstitutional, as argued by the Trump administration's Justice Department and the Republican coalition, the law's insurance coverage would end for an estimated 20 million people, including protection for people with pre-existing conditions, the Medicaid expansion in many states, coverage for young persons up to age 26 on their parents' plans, subsidies for low-income people and a host of other wide-ranging changes.

The Fifth Circuit's decision "poses a severe, immediate, and ongoing threat to the orderly operation of healthcare markets throughout the country, casts doubt over whether millions of individuals will continue to be able to afford vitally important care, and leaves a critical sector of the nation's economy in unacceptable limbo," House general counsel Douglas Letter told the justices. Letter is assisted by Munger, Tolles & Olson partner Donald Verrilli Jr. and Elizabeth Wydra of the Constitutional Accountability Center.

But U.S. Solicitor General Noel Francisco countered in his response  that the Fifth Circuit's decision did not "definitively" resolve any question of practical significance.

The appeals panel, Francisco told the justices, sent the case back to the district court to determine whether the mandate and other provisions could be severed from the law. The House's reason for speeding up proceedings, Francisco wrote, "at bottom, is that the vitality of the ACA's myriad provisions is too important to be left unresolved. But definitive resolution of that issue will be facilitated, not frustrated, by allowing the lower courts to complete their own consideration of the question."

The Justice Department argued initially that only those provisions closely tied to the mandate—for example, the requirement to sell coverage to people with pre-existing conditions and not to charge them more—should be struck down with the mandate. But when ACA defenders appealed, the administration changed its position to support the GOP challengers' argument that the entire law must fall.

The justices on Friday did agree to hear another Obamacare case, taking up a dispute over a Trump Administration rule expanding the types of employers who can claim religious exemptions from providing contraceptive coverage. A decision in that case is expected by June.

Read more: 

• Democrat AGs ask Supreme Court to fast-track ACA decision
• Uninsured rate rises, despite increase in employer-sponsored coverage
• 6 ways states are preparing if the ACA is invalidated