For the second time in four Supreme Court terms, Chief Justice John Roberts has broken with conservatives and rescued the Affordable Care Act from potential oblivion.

Roberts, an unlikely hero of sorts for the law, wrote the majority opinion on Thursday upholding a key provision of Obamacare that helps provide health-insurance subsidies to more than 6 million Americans.

“Congress passed the Affordable Care Act to improve health insurance markets, not to destroy them,” Roberts wrote in his opinion Thursday.

He was joined by the court’s four liberals — and, this time, traditional swing Justice Anthony Kennedy. It prompted a scathing dissent from fellow conservative Justice Antonin Scalia, who took a shot at Roberts.

This case, King v. Burwell, was long considered by observers to be a chance for Roberts to redeem himself with conservatives, who felt slighted by his decision to break with the court’s right-leaning members in 2012.

He reportedly changed his vote at the last minute, delivering the majority opinion that upheld the heart of the Affordable Care Act — its mandate that individuals buy insurance or pay a penalty. Roberts said the penalty was constitutional because it was the same thing as a tax.

The key question in this case centered on whether the federal government had the ability to provide subsidies to help low-income Americans buy health insurance.

The challengers in the case argued that the way the law was written does not allow for subsidized insurance in states where the federal government had set up insurance exchanges. Instead, the challengers argued, insurance subsidies are allowed only in states that have set up their own exchanges. They pointed to a clause that they argued meant exchanges should be “established by the state,” but members of Congress who were involved in writing the law disputed that characterization. Thirty-four states currently rely on the federal marketplace.

Roberts’ opinion suggested he viewed the wording of the law as ambiguous but thought the intent of Congress was clear. Congress, he wrote, always had the intention of subsidies being provided nationwide.

Reuters/Jim Bourg / President Barack Obama with Supreme Court Chief Justice John Roberts, left, as he arrives to deliver his State of the Union speech on Capitol Hill.

“Had Congress meant to limit tax credits to State Exchanges, it likely would have done so in the definition of ‘applicable taxpayer’ or in some other prominent manner,” Roberts wrote. “It would not have used such a winding path of connect-the-dots provisions about the amount of the credit.”

Supreme Court observers couldn’t figure out which way Roberts was leaning ahead of the decision, as he made only two substantive comments during the case’s oral arguments. That was as often as the number of times he opened his mouth to crack a joke at one of the lawyers involved in the case.

On one hand, observers said, he upheld the law last time. On the other hand, he’s viewed as generally pro-business in his decisions, and multiple businesses and organizations have issued briefs on the side of the administration in this case.

Roberts’ decision made clear that he subscribed to Solicitor General Donald Verrilli’s argument that getting rid of the subsidies might have led to a “death spiral” for states’ insurance markets.

“The combination of no tax credits and an ineffective coverage requirement could well push a State’s individual insurance market into a death spiral,” Roberts wrote, citing studies that predicted premiums could rise by as much as 47% while enrollment could decline by 70%.