Roberts tweaked Obama for not having "the courage of conviction" to continue enforcing the law even if he thought it was discriminatory.
At a news conference later Wednesday, House Democratic leader Nancy Pelosi of California called congressional Republicans irresponsible for funding the defense of what she called a discriminatory law at a cost she put at $3 million.
"On the basis of what I heard, the questions of the justices, the response of the participants, I'm very optimistic that DOMA will be struck down," Pelosi said.
"We didn't have horns"
To Windsor, Wednesday's arguments represented another step in the evolving history of gay rights in America.
For years, she said, she wore a circle of diamonds as a pin instead of a traditional diamond ring to hide her lesbian relationship from co-workers.
"We all lived, really, behind masks and in closets, indeed," Windsor explained. "So what happened is there was, as we increasingly came out, people saw that we didn't have horns. People learned that, okay, we were their kids and their cousins and their friends, all of whom were coming out for the first time. And I think it just, it just grew to where we were human beings like everybody else. And I really think that's what made the change."
Laughing, she told reporters: "I'm talking to you freely. I'd have been hiding in a closet 10 years ago."
Public interest remained high outside the court in advance of Wednesday's session, but not at the same level as Tuesday's arguments in another case involving California's voter-approved ban of same-sex marriage.
A smaller crowd gathered than the day before, with most of them opponents of DOMA.
"I'm here today because I'm a social worker and I've seen a lot of people suffer over the years," Mary Ann Piet told CNN. "And I'm concerned about not getting people their human rights, their dignity as people."
Conservative supporters of the law contend it codifies a fundamental cornerstone of society, and changing the definition of marriage would have widespread negative impacts.
The California case
During Tuesday's arguments, the justices seemed to lack consensus on both jurisdictional and constitutional questions relating to the voter-approved California law, known as Proposition 8, which bans same-sex marriage.
The overriding legal question in the California case is whether the Constitution's guarantee of equal protection under the law prevents states from refusing marriage to a defined class of people.
When it rules, the court could strike down laws across the country banning same-sex marriage, or it could leave the current patchwork of state laws in place, choosing to let state legislatures and state courts sort it all out.
"This was a deeply divided Supreme Court, and a court that seemed almost to be groping for an answer here," CNN Senior Legal Analyst Jeffrey Toobin said after the arguments.
Four of the more liberal justices seemed at least open to the idea that same-sex marriage should be allowed in California. Three of the more conservative justices seemed aligned with the view that marriage should only be for a man and a woman, and it's likely they'd be joined by Justice Clarence Thomas, who doesn't speak at arguments.
That could leave Kennedy as the swing vote, as has often been the case.
While admitting the law's defenders are "not just any citizens," Kennedy raised concerns about whether just the possibility of same-sex marriage was enough to establish they had suffered harm -- a key jurisdictional hurdle allowing them to appeal in the first place.
Nine states permit same-sex marriage
Among the 41 states that now forbid same-sex marriage, nine of them allow civil partnerships. Nine other states allow same-sex marriage, and about 120,000 same-sex couples have gotten married, according to estimates.
Prohibitions seem to run counter to polls that show rising support overall for same-sex marriage.
A CNN/ORC International poll released on Monday found 53% of Americans now support same-sex marriage, up from 40% in 2007. As to how the federal government should handle the issue, another CNN/ORC International poll out Tuesday found 56% of the public feels the federal government should also legally recognize same-sex marriages.