The US Supreme Court has heard final arguments on President Obama's healthcare bill, debating whether it could stand if a key measure is cut.
A lawyer for 26 states challenging the law said the rest of the bill was untenable if the legal requirement to buy health insurance was struck down.
The justices have heard three days of argument on the Affordable Care Act, the longest such debate in years.
A decision could come in June, midway through the presidential campaign.
On Wednesday the law's challengers told the court that, without the so-called individual mandate, the law could not function in the way Congress intended and thus the whole bill must be scrapped.
The government argued that if the insurance mandate was quashed, two other provisions that depend on the mandate would have to go too - but most other elements of the 2,400-page law should remain.
The two related provisions are a requirement to offer coverage to people with pre-existing medical conditions, and limits on how much insurance companies can charge in premiums based on a person's age or health.
The Supreme Court has released audio recordings and written transcripts of Wednesday's proceedings.
A step beyond?
Some justices appeared to question whether it would be possible to redeem the bill if its core provision was struck down.
Justice Antonin Scalia, a conservative who on Tuesday questioned the legality of the individual mandate, said: "My approach would be to say that if you take the heart out of this statute, the statute's gone."
But Justice Elena Kagan - appointed to the court by President Obama - said that healthcare systems without a mandate operated well in states like Utah. "Is half a loaf better than no loaf?" she asked.
Meanwhile, Justice Ruth Bader Ginsberg said that if the coverage requirement was deemed unconstitutional the court would have to decide between a "wrecking operation" and a "salvage job".
A private lawyer, H Bartow Farr III, was appointed by the judges to argue a third point of view: the rest of the law could proceed in the absence of the mandate.
In August last year, a federal appeals court in Atlanta ruled that the insurance requirement was unconstitutional, but said the rest of the law could remain in place.
The court also set aside an hour in the afternoon to consider the states' challenge to the expansion of the Medicaid programme for low-income and disabled Americans.
Afterwards, the White House said it was "confident" that the landmark healthcare law was constitutional.
Deputy Press Secretary Josh Earnest said the model of the individual mandate was "bipartisan" and had originally been supported by Republicans.
"This was a novel policy solution that... was promoted by conservative Republicans in Washington DC, as a solution to difficult healthcare challenges," he said.
The law, passed in March 2010 and the most significant reform to US healthcare in almost half a century, aims to extend health insurance to 30 million Americans.
But its requirement that all those eligible should have medical cover has been condemned as an assault on civil liberties by conservatives.
On Tuesday, the justices spent two hours grilling lawyers about the individual mandate, which would require eligible Americans to hold health insurance or face a penalty.
Justice Anthony Kennedy, a conservative regarded as a possible swing vote on the bench, asked whether the measure was a "step beyond what our cases allow".
But he conceded that removing the requirement for all Americans to be covered would affect "the rates of insurance and the costs of providing medical care in a way that is not true in other industries".
The conservative Chief Justice John Roberts appeared sceptical of the argument by the law's challengers that if the federal government can require people to purchase health insurance, it can also make them buy cars.
"Everybody is in this market, so that makes it very different than the market for cars or the other hypotheticals that you came up with, and all they're regulating is how you pay for it," he said.
The justices are expected to meet in private on Friday to discuss the arguments made during the hearings. They may also conduct a preliminary vote on how they intend to rule on the case.
Each justice will then draft a written opinion on the case.
There are nine Supreme Court justices - five appointed by Republican US presidents and four by Democrats.
The law can survive if just one of the conservative appointees joins the four liberals on the bench, but if the conservatives remain unified, the law would be overturned.