The nation's highest court is hearing Obergefell v. Hodges, a case that examines if same-sex couples have a constitutional right to marry. Here's what you need to know about the case that could make gay marriage legal across the nation. (Julie Percha/The Washington Post)

The Supreme Court’s historic consideration Tuesday of whether the Constitution protects the right of same-sex couples nationwide to marry seemed to come down to a familiar arbiter: Justice Anthony M. Kennedy.

That’s normally a safe haven for gay rights activists — Kennedy has written each of the court’s major victories advancing their movement. But the question after the hearing seemed to be whether forcing reluctant states to allow same-sex unions was a logical extension of the court’s rulings or too much, too fast.

Kennedy seemed to be working it out. On the one hand, he pressed lawyer Mary L. Bonauto, representing gay couples challenging states’ bans, to explain why the court should change the tradition of marriage as only between a man and a woman when the concept of same-sex marriage is so new.

“This definition [of traditional marriage] has been with us for millennia,” Kennedy said. “And it’s very difficult for the court to say, ‘Oh, well, we know better.’ ”

But by the end of the arguments, questioning John Bursch, the attorney representing four states that want to keep restrictive laws, Kennedy sounded more like one of the lyrical passages in one of his opinions.

Same-sex marriage status in the U.S., state-by-state

“Same­-sex couples say, of course, we understand the nobility and the sacredness of the marriage,” Kennedy told Bursch. “We know we can’t procreate, but we want the other attributes of it in order to show that we, too, have a dignity that can be fulfilled.”

The justices are considering two simple-sounding questions: whether the Constitution requires states to issue marriage licenses to same-sex couples and, if not, whether states must recognize same-sex marriages performed in other states where they are legal.

But the arguments were filled with discussions of equal protection and fundamental liberties, how an understanding of the Constitution changes with society, and when majority rule must give way to minority rights.

Chief Justice John G. Roberts Jr., the member of the court who most seemed during arguments to be searching for middle ground, said a decision finding a constitutional right at this time would mean radical change and would short-circuit public debate.

“You’re not seeking to join the institution — you’re seeking to change what the institution is,” Roberts told Bonauto. “The fundamental core of the institution is the opposite-sex relationship, and you want to introduce into it a same-sex relationship.”

Roberts added later: “If you prevail here, there will be no more debate. I mean, closing of debate can close minds, and it will have a consequence on how this new institution is accepted. People feel very differently about something if they have a chance to vote on it than if it’s imposed on them by the courts.”

He also pointed to a different way to look at the case, rather than casting it as discrimination based on sexual orientation.

“I mean, if Sue loves Joe and Tom loves Joe, Sue can marry him and Tom can’t,” Roberts said. “And the difference is based upon their different sex. Why isn’t that a straightforward question of sexual discrimination?”

Bonauto wanted a clear ruling on the constitutional question. She said it was the states that had closed the debate, by passing constitutional amendments defining marriage as only between a man and a woman. Amending state constitutions and relying on majority vote for rights is not what the Constitution anticipates, she said.

“If a legal commitment, responsibility and protection that is marriage is off limits to gay people as a class, the stain of unworthiness that follows on individuals and families contravenes the basic constitutional commitment to equal dignity,” Bonauto said.

She was making her first argument before the Supreme Court but has been a pioneer in the legal movement to secure same-sex marriage, winning the first major case that found a right to same-sex marriage, in Massachusetts in 2003.

Bonauto received a boost from Justice Ruth Bader Ginsburg, who pointed out that definitions of marriage had already changed. “Marriage was a relationship of a dominant male to a subordinate female,” she said.

But there was resistance from the conservative justices, and a bit from one of the liberals, Justice Stephen G. Breyer. “Suddenly, you want nine people outside the ballot box to require states that don’t want to do it to change.”

Justice Samuel A. Alito Jr. asked that if the definition of marriage was simply a commitment between loving consenting adults, how could a state withhold that from siblings, or two women and two men who decided to marry. “Would there be any ground for denying them a license?” Alito asked.

Justice Antonin Scalia said that if the decisions on marriage continue to be made democratically by the states, those states could make religious accommodations that would not be possible if there was a decision that same-sex marriage is a constitutional right.

“Is it conceivable that a minister who is authorized by the state to conduct marriage can decline to marry two men if indeed this court holds that they have a constitutional right to marry?” he asked.

Bonauto said it was well established that clergy are not forced to perform actions that violate their religious beliefs.

The Obama administration weighed in on behalf of the couples, and Solicitor General Donald B. Verrilli Jr. argued that withholding marriage violates equal-protection guarantees.

“In a world in which gay and lesbian couples live openly as our neighbors, they raise their children side by side with the rest of us, they contribute fully as members of the community . . . it is simply untenable — untenable — to suggest that they can be denied the right of equal participation in an institution of marriage, or that they can be required to wait until the majority decides that it is ready to treat gay and lesbian people as equals,” he said.

Bursch, a former Michigan solicitor general representing that state along with Ohio, Kentucky and Tennessee, pleaded with the justices to allow the democratic debate over same-sex marriage to continue.

“This case isn’t about how to define marriage,” he said. “It’s about who gets to decide that question. Is it the people acting through the democratic process or is it the federal courts? And we’re asking you to affirm every individual’s fundamental liberty interest in deciding the meaning of marriage.”

Justice Sonia Sotomayor stopped him there. “Nobody is taking that away from anybody,” she said. “Every single individual in this society chooses, if they can, their sexual orientation or who to marry or not marry.”

Bursch said that the states’ interest in marriage was not to “deny dignity or to give second-class status to anyone. It developed to serve purposes that, by their nature, arise from biology.”

He said the states’ interest in marriage was keeping together men and women to care for the children that they intentionally or accidentally create.

Justice Elena Kagan asked Bursch if he believes that “if one allowed same-sex marriage, one would be announcing to the world that marriage and children have nothing to do with each other.”

He said the state has a child-centric interest in marriage, not to legitimize the relationships of committed adults.

Roberts dominated the second argument, about whether states could be forced to recognize marriages performed in states where they were legal.

The question would be moot if the court declares a constitutional right, but the second argument lent force to the idea that it might be the chief justice’s preferred path and could perhaps win a wider majority.

If states are forced to recognize same-sex marriages performed elsewhere, Roberts suggested, it would be “only a matter of time” before same-sex marriage settled in as a national norm. It would effectively allow “one state” or a minority of states to “set policy for the nation.”

At the same time, the Roberts line of questioning suggested he did not view that possibility with great alarm, at least as an alternative to a court decision holding that all states must permit same-sex marriages within their borders.

Isn’t it “quite rare for a state not to recognize” a marriage performed elsewhere? he asked.

The recognition of a constitutional right would mark the culmination of an unprecedented upheaval in public opinion about gay rights and a dramatic change in the nation’s jurisprudence. Same-sex marriages were practically unheard of in the nation until a Massachusetts court decision cleared the way for unions there just a dozen years ago.

Now, more than 70 percent of Americans live in states where same-sex couples are allowed to marry, according to estimates.

The questions raised in the cases that the court will consider were left unanswered in 2013, when the justices last confronted the issue of same-sex marriage. A slim majority of the court said at the time that a key portion of the federal Defense of Marriage Act — withholding recognition of same-sex marriages — was unconstitutional and in a separate case allowed same-sex marriages to resume in California.

Since then, courts across the nation — with the notable exception of the Cincinnati-based federal appeals court that left intact the restrictions in the four states at issue — have struck down a string of state prohibitions on same-sex marriage, many of them passed by voters in referendums.

When the Supreme Court declined to review a clutch of those court decisions in October, same-sex marriage proliferated across the country.

Couples may now marry in 37 states and the District of Columbia.

Public attitudes toward such unions have undergone a remarkable change as well. A recent Washington Post-ABC poll showed a record 61 percent of Americans say they support same-sex marriage. The acceptance is driven by higher margins among the young.

When the justices declined in October to review the string of victories same-sex marriage proponents had won in other parts of the country, it meant the number of states required to allow gay marriages grew dramatically, offering the kind of cultural shift the court often likes to see before approving a fundamental change.

The combined cases now before the Supreme Court are Obergefell v. Hodges.

Mark Berman and Sandhya Somashekhar contributed to this report.