Correction: An earlier version of this story inaccurately reported that federal courts struck down same-sex marriage bans in 22 states. The number is 21; New Jersey should not have been included among those states. The story has been corrected.

The nation's highest court is hearing Obergefell v. Hodges, a case that examines whether 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)

When a federal judge declared same-sex marriage legal in Florida last year, it should have changed the way Bruce Stone does his job. The estate-planning lawyer had for years helped gay couples patch together legal documents to try to approximate some of the protections enjoyed by heterosexual spouses.

But with the Supreme Court set to decide later this year whether that court decision and others ought to stand, Stone isn’t taking any chances. He is still writing up power-of-attorney forms and setting up trusts out of state, and he has some stark advice for his gay clients: “Do not get married here in Florida.”

On Tuesday, the Supreme Court is set to hear arguments on whether gay couples have a constitutional right to get married. But if the court rules against that right, the ability to decide reverts to the states, and Florida and others might just slam the door.

[Conservative justices sharply question lawyers for same-sex couples]

Gay rights advocates have publicly proclaimed their confidence that the court will go their way, and many opponents reluctantly agree. They think the court has telegraphed its intention to establish a broad new legal right by allowing marriages to proceed in several states where the right is contested.

But the possibility that the court may defy expectations, as it has so many times before — and the enormous consequences if it does — have caused people like Stone to make contingency plans in case the recent momentum in favor of same-sex marriage is suddenly reversed.

“It would be a mess,” said Dale Carpenter, a professor of constitutional law at the University of Minnesota, noting that marriage confers 1,100 rights and benefits at the federal level and hundreds more from the states, from filing taxes jointly to inheriting hunting licenses. “There would be great uncertainty in the aftermath of such a ruling. All kinds of possibilities we can’t even think of would arise.”

The effect would be explosive in the 21 states where same-sex-marriage bans were struck down by federal courts. Groups for and against these unions say such a decision would set off a cascade of fresh litigation and spark dramatic new fights in state capitals, with each side jockeying to have its version of marriage enshrined in state law.

Some legal experts say the old laws in those states would snap back into place, immediately shutting the door on future marriages, while others contend that would require another round of litigation. Some believe the thousands of marriages that have taken place in those states would be deemed valid, though others think the matter would need to be settled by the courts — perhaps even the Supreme Court.

States’ different reactions

The justices could specify how states should treat their ruling. But the precise fallout would probably depend on the state.

In deep-red states such as Oklahoma, Utah and Kansas, officials probably would waste no time trying to put a stop to same-sex marriages. Groups may attempt to have existing marriages invalidated or may press state officials not to allow state benefits for gay couples who have wed.

Arizona state Sen. Steve Smith, a Republican from Maricopa County, predicted an immediate push to reinstate a constitutional amendment, approved in a 2008 voter referendum, defining marriage as between a man and a woman. “I don’t know how much clearer the will of the people can be expressed than by a vote to that effect,” he said.

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

In states such as Oregon, where the political climate has become more favorable to gay marriage in recent years, there probably would be a scramble to enact legislation to allow same-sex marriages.

But the process could be more drawn out in places such as California, whose prohibition on same-sex marriage was part of the state constitution. If that ban was reinstated as a result of a Supreme Court decision, a voter referendum would be needed to get rid of it.

“People would be ready to fight,” said David Fleischer, a gay rights activist in Los Angeles.

Elsewhere, the battles could be more pitched. In Virginia and Pennsylvania, for instance, freshly minted Democratic governors may resist attempts to revert to old laws, potentially clashing with conservative state lawmakers. And Republican leaders in Florida and elsewhere could find themselves squeezed between their conservative bases and gay rights forces that would label them bigots.

Social conservatives are split on how significant a decision favorable to their side would be. Some say it would be a victory on its own, while others stress that their goal is a national standard defining marriage as exclusively between a man and a woman. The justices are not considering whether to set such a standard.

A win at the Supreme Court this year would be “a big victory, but in terms of the battle it’s far from over,” said Mat Staver, founder of Liberty Counsel, a law firm that defends conservative Christian values. “It means there’s a lot of work to be done.”

The effect would be more muted in the District and 16 states that made same-sex marriage legal through the actions of local legislators and judges. At the other end of the spectrum, bans have remained in place in 13 states because federal judges did not invalidate them or they stayed decisions overturning the bans. The laws wouldn’t change in these states in the wake of the court’s decision, but statehouse battles could nevertheless erupt as activists leap into action.

Uncertainty for couples

The biggest question: What would become of the thousands of couples who got married in the 21 states during the brief period same-sex marriage was allowed?

James Esseks, an attorney for the American Civil Liberties Union, pointed to several recent cases involving same-sex marriage that suggest courts generally think that “once you’re married, you’re married.”

But some experts think it could take years of litigation, and perhaps another go before the Supreme Court, to clarify that.

Already, lawyers tend to recommend a cautious path for same-sex couples. A gay couple married in Iowa, for example, may cross the border into Nebraska and find that their union is not recognized. As a result, lawyers suggest that couples have power-of-attorney paperwork in place to ensure their ability to make decisions for each other in an emergency.

And they recommend that gay non-biological parents formally adopt their children — even if the parent’s name already appears on the child’s birth certificate.

Stone recommends that his gay clients get married in New York or another state where the right to same-sex marriage was established through state action. That way, even if the couple’s relationship isn’t recognized in Florida, it remains valid in the eyes of the federal government.

Carla Petersen, who married her partner of 26 years in Florida in January, said she is accustomed to uncertainty. “It won’t make it any worse than it ever was before,” said Petersen, 65, a resident of Venice, Fla.

Before marrying, Petersen and her partner, Sharon Van Butsel, 70, signed power-of-attorney documents for each other and made sure their assets are held in both their names. But they couldn’t get Social Security benefits, which they now enjoy as a formally married couple. And they struggled to explain their relationship to the perplexed receptionist at the doctor’s office.

If the validity of their marriage comes into question, they could lose those Social Security benefits. But Petersen doubts she will ever again have to deal with confusion at the doctor’s office.

“Times have really changed. The acceptance is there,” she said. “Ours has become the right side.”