Supreme Court Justice Anthony Kennedy emerged as a pivotal vote as the court weighed whether to strip more than 7 million Americans of the federal subsidies that helped them buy health insurance.
At several points during Wednesday's hearing, Kennedy said the challengers' reading of the 2010 statute could violate states' rights by coercing them to set up insurance marketplaces.
"If your argument is accepted, the states are being told either create your own exchange, or we'll send your insurance market into a death spiral," Kennedy said to Michael Carvin, the lawyer representing the challengers.
Kennedy's questions suggested he was at least a potential fifth vote to back the administration's argument that the law provides the credits to purchasers in all 50 states.
But Kennedy also suggested several times that the challengers' reading of the law was the more natural one. "Perhaps you will prevail on the plain words of the statute," he told Carvin.
Even so, the thrust of Kennedy's questions opened up a new path for the administration to win the case.
"He is the big question mark," said Cory Andrews, a lawyer with the Washington Legal Foundation, which backs the challenge against the Affordable Care Act. "That's the big takeaway."