One exchange lingered in the hours after the U.S. Supreme Court heard arguments Wednesday on the constitutionality of the Defense of Marriage Act (DOMA), the federal law that prohibits the federal government from recognizing same-sex marriages or providing same-sex spouses with federal benefits.

As Solicitor General Donald Verrilli took the podium on behalf of the Obama administration, prepared to argue that the law violated the equal protection rights of gay couples, Chief Justice John Roberts quickly interjected. He pressed Verrilli to explain the role of state rights in the DOMA case, hinting that if the federal government acknowledged gay marriage, it could infringe on the sovereignity of the 30-plus states that have banned those unions.

“You agree that Congress could go the other way, right?" Roberts asked. "Congress could pass a new law today that says, ‘We will give federal benefits. When we say "marriage" in federal law, we mean committed same-sex couples as well, and that could apply across the board,’” Roberts said. “Or do you think that they couldn't do that?”

“We think that wouldn't raise an equal protection problem like this statute does, Mr. Chief Justice,” Verrilli replied, trying to pivot back to his main argument. Roberts didn’t let the matter go.

“Well, no, my point is: It wouldn't -- you don't think it would raise a federalism problem either, do you?” Roberts said.

“I don't think it would raise a federalism problem,” Verrilli said. He attempted to move forward with the equal protection argument that the White House had presented in its briefs, but Roberts cut in one more time.

“So just to be clear, you don't think there is a federalism problem with what Congress has done in DOMA?” Roberts asked again.

“No, we don’t,” Verrilli said.

Why is this exchange so important? Because of what happened a few minutes before Verrilli began his argument. Justice Anthony Kennedy, considered the likely swing vote in the DOMA case and a staunch believer in state rights, had spent several minutes grilling Verrilli’s opponent, Paul Clement, who was representing supporters of the federal law.

Kennedy sounded skeptical of Clement’s assertion that DOMA didn’t violate state rights even though it stopped the federal government from recognizing same-sex marriages in the nine states and the District of Columbia that had decided to legalize them. After all, the federal government has traditionally deferred to an individual state’s definition of marriage. At one point, Kennedy said: “You are … at real risk of running in conflict with what has always been thought to be the essence of the state police power, which is to regulate marriage, divorce, custody.”

Seasoned Court watchers will tell you that the real point of oral arguments is for the nine justices to debate each other. They already know each side’s legal argument. But through the questions they ask the attorneys, they can try to persuade their colleagues on the bench to see things differently.

From that perspective, it’s reasonable to view the persistence on the federalism question from Roberts, who is generally expected to uphold DOMA, as an effort to sway Kennedy to his side by appealing to the latter’s longstanding advocacy for state rights. Maybe DOMA doesn't have anything to do with state rights, Roberts seemed to suggest. Or if it does, the matter cuts both ways.

Roberts might have made his point. After he appeared to let the issue go and Verrilli began to make his equal protection case, Kennedy interrupted.

“You think Congress can use its powers to supersede the traditional authority and prerogative of the states to regulate marriage in all respects? Congress could have a uniform definition of marriage that includes age, consanguinity, etc., etc.?” he asked Verrilli.

"No, I'm not saying that, Your Honor. I think if Congress passed such a statute, then we would have to consider how to defend it. But that's not—” Verrilli said before Kennedy cut him off.

“Well, but then there is a federalism interest at stake here, and I thought you told the Chief Justice there was not,” Kennedy said.

“Well, with respect to Section 3 of DOMA, the problem is an equal protection problem from the point of view of the United States,” Verrilli said.

Justice Elena Kagan, one of the Court’s liberals, then intervened. By her wording, she seemed to be reemphasizing—maybe for Kennedy’s sake—that the states, not the federal government, had historically defined what marriage was.

“Yes but, General, surely the question of what the federal interests are,” Kagan said, “and whether those federal interests should take account of the historic state prerogatives in this area is relevant to the equal protection inquiry?”

“It's central to the inquiry, Justice Kagan. I completely agree with that point,” Verrilli said.

Roberts then seized on the Solicitor General’s apparent concession that there is a federalism issue in the DOMA case. He re-raised his concern that there would be ramifications for state rights however the Court rules on the law by again proposing his hypothetical of Congress recognizing same-sex unions nationwide.

“Oh, so it would be central to the inquiry if Congress went the other way, too?” Roberts asked.

“Well, the difference is what (DOMA) does is impose this exclusion from federal benefits on a class that has undeniably been subject to a history of terrible discrimination on the basis of—” Verrilli said, trying to keep the focus on equal protection, before Roberts stopped him.

“I understand that. That's your equal protection argument. It's not very responsive to my concern I'm trying to get an answer to,” Roberts said.

He then hammered his point home, seeking to trap Verrilli into admitting that the federal government's recognizing same-sex marriages has implications for state rights as much as the federal government's ignoring them does. “You don't think federalism concerns come into play at all in this, right?”

Verrilli failed to give a full answer to that question, but the Chief Justice's point was made.

Now the question is whether the exchange left an impact on Kennedy. The four conservative justices are expected to uphold DOMA. The four liberal justices are expected to strike it down. Kennedy holds the fifth vote for either side. Parsing oral arguments for signs of how the justices will rule is a difficult exercise.

But it’s easy to imagine that Kennedy’s ultimate decision will depend on how he interpreted these points, whether he maintains the opposition to DOMA that he expressed to Clement—or whether Roberts managed to convince him that overturning DOMA would impede state rights as much as upholding it would.