The Supreme Court on Wednesday concluded its historic two-day scrutiny of the thorny issue of same-sex marriage, displaying wariness about ruling on the subject even as it appeared possible that the justices will strike down the federal Defense of Marriage Act.

With hundreds of partisans on both sides parading and debating in front of the court on Tuesday and Wednesday, the justices inside probed all aspects of the issue from the philosophical and political to the procedural.

On Tuesday in Hollingsworth v. Perry, justices’ concerns focused on the impact of same-sex marriage on society and children. In United States v. Windsor, the Wednesday case, the court had more practical worries about whether the case could proceed at all under the constitutional requirement that the court decide only "cases and controversies." With the Justice Department no longer defending DOMA, and a group of House members standing in its place, several justices wondered if the case was justiciable in its present form.

Plaintiff Edith Windsor, who was in the courtroom Wednesday, challenged Section 3 of the 1996 law, defining marriage as between a man and a woman for federal purposes. She claimed that in spite of her marriage to a woman—legally recognized in New York, where she lives—the law required her to pay a steep federal estate tax when her spouse Thea Spyer died. Opposite-sex married couples are spared the tax. In 2011, the Obama administration announced it would no longer defend the statute in cases like Windsor’s. A Republican-led House group, called the Bipartisan Legal Advisory Group (BLAG), intervened to defend the statute.

Allowing BLAG to defend the law instead of the United States is "very strange," in the words of Justice Sonia Sotomayor. Justice Antonin Scalia said courts should not decide cases "just for fun" when the original parties in litigation end up on the same side. And Chief Justice John Roberts Jr. said that asking the court to decide a case in which "all parties agree" is "something we’ve never done before."

Harvard Law School professor Vicki Jackson, appointed by the court to argue that the court has no jurisdiction, made a strong case that BLAG’s participation was "inconsistent with separation of powers," because once a law is passed, Congress’s powers are "at an end." She said the court should resist the "natural urge" to decide the case now and should await another case.

Justice Elena Kagan suggested that no matter what its stance on the law, the government can claim it will suffer an injury – the loss of Windsor’s estate tax payment—thereby triggering jurisdiction. But Jackson said "the government has not asked this court to overturn the injury,’ canceling that justification for jurisdiction.

Deputy Solicitor General Sri Srinivasan made headway when he told the court it has jurisdiction anyway, in part because the government has continued to enforce the law in spite of its stance "out of respect" for Congress.

But justices seemed far more dubious about giving standing to BLAG to substitute for the government. Former solicitor general Paul Clement, now a partner at Bancroft, argued on behalf of BLAG that it was entitled to participate in the case to vindicate the interest of House members to see that the law they passed is enforced. Justice Sonia Sotomayor asked, "Then why shouldn’t taxpayers have a right to come in? And we say they don’t."

After 50 minutes of discussion of the standing issues, the court turned to the merits of whether the federal law violates "equal protection" guarantees by treating legally married same-sex couples differently than their opposite-sex counterparts.

Clement argued that Congress is entitled to establish uniform rules for federal benefits—even if they differ from state definitions. "It’s much more rational," he said, "for Congress to say, we want to treat the same-sex couple in New York the same way as the committed same-sex couple in Oklahoma," where gay marriages are not recognized. By staying uniform across the nation, Clement also said the law also has the benefit of keeping the federal government, "a big player," out of the state-by-state debate over same-sex marriage.

Justice Ruth Bader Ginsburg immediately challenged Clement, asserting that the federal law has such a pervasive effect on taxes, Social Security and family leave, among other issues, that same-sex couples are entitled to wonder, "What kind of marriage is this?" Later, she said DOMA consigns states to offering "two kinds of marriage: real marriage and then this sort of skim milk marriage."

Justice Stephen Breyer also criticized Clement’s stance: "You’re saying uniform treatment is good enough no matter how odd it is, how irrational." Another skeptical justice was Sotomayor, who said, "what gives the federal government the right to be concerned at all at what the definition of marriage is?"

Asserting that DOMA was passed in 1996 at least in part out of animus toward homosexuality, Justice Kagan read from the legislative history, which said "moral disapproval of homosexuality" was one of the reasons for the statute. Clement said that alone is not a sufficient reason to strike down the law under "rational basis" scrutiny of such laws.

Justice Anthony Kennedy, widely viewed as the pivotal vote on the issue, also said the pervasive nature and lure of federal benefits, denied by DOMA to legally married same-sex couples, poses "the 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."

Given the critical comments about DOMA made by Justices Ginsburg, Breyer, Sotomayor and Kagan, Kennedy could provide a fifth vote to strike down the law.Judith Schaeffer, vice president of the liberal Constitutional Accountability Center said, "A majority of justices today appeared to regard DOMA for what it is: a pervasive program of discrimination that changes 1100 laws and touches every aspect of life."

But the argument was not yet over. Solicitor General Donald Verrilli Jr. rose to attack the law that he would ordinarily have been tasked with defending. He told the justices unequivocally that "Section 3 discriminates," was motivated by "moral disapproval," and gives lesser status to a group has been "subject to a history of terrible discrimination."

Several justices tried to draw Verrilli into a discussion of how the law affects federalism—the balance between state and federal powers—but Verrilli steered the debate back to equal protection.

Finally it was time for Roberta Kaplan of Paul, Weiss, Rifkind, Wharton & Garrison, Windsor’s lawyer, to talk about the practical impact of DOMA. Not only did it affect her client’s taxes, Kaplan said, but it influences a wide range of other laws – including campaign finance statutes, conflict of interest laws and judicial recusal statutes where same-sex married couples are free, ironically, to do what opposite-sex couple may not in the area of campaign contributions and gifts or employment that would otherwise trigger ethics laws

When Roberts asked Kaplan a federalism question and she, like Verrilli, tried to steer it back to equal protection, he cut her off. "You’re following the lead of the solicitor general," he said sharply.

Roberts also challenged the view that Congress acted with animus toward homosexuals when it passed DOMA in 1996. "Times can blind," said Kaplan, "and back in 1996 people did not have the understanding that they have today, that there is no … constitutionally permissible distinction."

Justice Antonin Scalia chimed in. "Why are you so confident in your judgment?" he said, noting that only nine states permit same-sex marriage."With respect to the understanding of gay people and their relationships there has been a sea change," Kaplan replied.

That prompted Roberts to try one more time to deflate Kaplan’s case. If there is has been a sea change, Roberts said, isn’t it the result of the "politically powerful" lobby in favor of same-sex marriage. "As far as I can tell, politicians are falling over themselves to endorse your side of the case."

He was trying to lead her into a trap, because political powerlessness is one of the factors the court uses in deciding what level of scrutiny to use to evaluate laws that disfavor categories of people. The more powerless the group, the harder it is for government to justify discrimination.

Kaplan steered clear, insisting that "no other group in recent history has been subjected to popular referenda to take away rights." She added, "I don’t believe that societal understanding came strictly through political power."

Tony Mauro can be contacted at tmauro@alm.com.