Chief Justice John Roberts and Justice Brett Kavanaugh each suggested they were inclined to favor "severing" the so-called individual mandate from the rest of the ACA, which has provided affordable medical coverage to tens of millions of Americans over the past decade.
In the case, California vs. Texas, the high court is seeking to determine whether eliminating the mandate -- the part of the law that required all uninsured Americans to buy health insurance through ACA exchanges or pay an income tax penalty -- renders the entire healthcare law, known colloquially as Obamacare, unconstitutional.
Kavanaugh, considered a swing vote on the issue, suggested the ACA could still pass constitutional muster without the mandate.
In an exchange with an attorney defending the law on behalf of the House of Representatives, Kavanaugh said, "I tend to agree with you this a very straightforward case for severability under our precedents, meaning that we would excise the mandate and leave the rest of the act in place."
Roberts later said he disagreed with a coalition of 18 Republican-led states headed by Texas Attorney General Ken Paxton, who argue the ACA was rendered unconstitutional by President Donald Trump's 2017 Tax Cuts and Jobs Act, which removed the penalty and declared it illegal.
"It's hard for you to argue Congress intended the entire act to fall if the mandate was struck down if the same Congress that lowered the tax penalty to zero did not even try to repeal the rest of the act," he told an attorney representing Paxton.
Some experts and observers consider the matter the most important case the Supreme Court will decide this term, which began last month and ends at the end of June. Striking down the ACA would cost as many as 20 million Americans their medical coverage and bring a tax cut to the wealthy.
Trump has been trying to repeal the ACA since he stepped into office -- while President-elect Joe Biden, who helped craft the 2010 law, said during his campaign that he would enhance and expand it.
Speaking Tuesday after arguments in the Supreme Court, Biden said that if the court strikes down the law during a pandemic, it would be "simply cruel and needlessly divisive."
"Let's be absolutely clear about what's at stake: The consequences of the Trump administration's argument are not academic or an abstraction. For many Americans, they are a matter of life and death, in a literal sense," he said. "This isn't hyperbole. It's real -- as real as it gets."
Biden said American families are "reeling" from the effects of the coronavirus pandemic.
"We have a moral obligation to ensure that healthcare is a right for all, not a privilege for a few.
"In the middle of a deadly pandemic that's affecting more than 10 million Americans, these ideologues are once again trying to strip health coverage away from the American people."
The part of the ACA that required uninsured people to buy coverage has helped the law stand up to legal scrutiny in the past. The Supreme Court upheld the law's constitutionality in a 2012 ruling, in which Roberts said the mandate was within Congress' taxing authority.
However, the 5th U.S. Circuit Court of Appeals last year upheld a lower court ruling that said the individual mandate was unconstitutional as "it can no longer be read as a tax and there is no other constitutional provision that justifies this exercise of congressional power."
The Democratic-led House of Representatives and a group of 20 Democratic states have asked the Supreme Court to overturn the lower court decisions, arguing that the Trump administration's stripping of the individual mandate had another impact on the case -- it made the tax issue entirely moot.
"[The law] may encourage Americans to buy insurance, but it does not require anyone to do anything," they wrote in their brief. "Individuals still have a choice: Buy insurance or don't."
In addition to deciding if the individual mandate is unconstitutional, the court will also have to decide whether the provision invalidates the law as a whole. Republicans and the Texas-led coalition say it does.
They argue that Congress intended for the ACA to work as an integrated whole. The Democrat-led group, however, counters that Congress consciously decided to leave the rest of the landmark law intact three years ago when it agreed to remove the individual mandate.
In previous Supreme Court rulings on the ACA, Justices Clarence Thomas and Samuel Alito Jr. said the law could not be separated or salvaged from the unconstitutional portions. Roberts and Kavanaugh, Trump's second high court appointee, noted that the court must closely examine whether the remainder of the law can be severed from the unconstitutional part and remain in place.
"Constitutional litigation is not a game of gotcha against Congress, where litigants can ride a discrete constitutional flaw in a statute to take down the whole, otherwise constitutional statute," Kavanaugh wrote.
The Supreme Court, which had just eight members for weeks after the death of Justice Ruth Bader Ginsburg, is back to full membership after the controversial addition of Amy Coney Barrett last month. She wrote in a law review article three years ago that she considered Roberts' decision eight years ago that affirmed the ACA "pushed" the law beyond its "plausible meaning" to save the statute.
During confirmation hearings, Barrett answered ACA-related questions from Democrats by saying "presumption is always in favor of severability."
"When it comes to whether thousands of lawful provisions of the ACA should fall because of one (purportedly) unconstitutional provision, the stakes could not be higher," wrote legal expert Pratik Shah, who filed an amicus brief on behalf of America's Health Insurance Plans, a trade association representing health insurance providers, in support of California and the Democratic-led House of Representatives.
"At the same time, the answer could not be easier. Scholars and court watchers from all perspectives agree: Whether or not the individual 'mandate' is unconstitutional, the rest of the ACA should stand. Even some of the federal government's most respected career lawyers, including from the solicitor general's office, appear to have been unwilling to associate themselves with the government's ultimate failure to defend the ACA."
Others argue that the mandate was illegal and that, as a result, the law should be illegal.
"If the individual mandate is unconstitutional, then the law can be saved only by severing its unconstitutional provision," argues expert Andy Schlafly, who filed an amicus brief in support of the Texas-led coalition.
"The Supreme Court should not create severability out of thin air in order to salvage a misguided law."
The high court heard Tuesday's oral arguments by teleconference, and a final decision is expected by next spring.