The arc of the moral universe is long, said abolitionist Theodore Parker. “My eye reaches but little ways. . . . And from what I see I am sure it bends toward justice.”

But few advocates of social change — in Parker’s time or ours — act as though justice really is inevitable. They try to hasten it.

So it is for advocates of marriage equality, who, against once-impossible odds, have emerged from the political and legal wilderness.

Nine states and the District, encompassing about 15 percent of the U.S. population, have legalized gay marriage. This includes three states whose voters approved it on Nov. 6. Though it was opposed by a clear majority, 57 percent to 35 percent, in a 2001 Pew Research Center poll, gay marriage now enjoys a 48 percent plurality.

And the Supreme Court has agreed to consider overturning both the federal Defense of Marriage Act and state bans on gay marriage, raising the prospect of a national victory — but also risks.

The court could reject the advocates’ arguments, setting a negative precedent. Or it could accept them — triggering a backlash even sharper than the pro-death-penalty reaction to the court’s attempt to strike down 40 state capital punishment laws in 1972.

Gay marriage may be routine in Massachusetts, but it remains unthinkable in Oklahoma. Thirty-three states ban it, by statute or by constitutional amendment — many of which were adopted by referendum in the past dozen years.

Such concerns explain why many gay rights lawyers frowned on the filing of a federal lawsuit to overturn California’s 2008 referendum banning gay marriage.

They also explain why even supporters of gay marriage might not want the Supreme Court to precipitate the matter.

A ruling that the Constitution prohibits defining marriage as the union of a man and a woman would be one of the most activist in history, sweeping aside dozens of democratically enacted state and federal laws.

If it were as obvious that laws enshrining traditional marriage represented the same evil that slavery or racial segregation did, and that the Constitution prohibits them as clearly as it prohibited official racism, then the right of same-sex couples to marry should be protected from the vagaries of majority rule in the states, whatever the consequences. Let justice be done, though the heavens fall.

But this case is not a no-brainer, for either side. At least I don’t think the lapidary phrases of our Constitution contain a definitive answer. And the court has had mixed results at slicing similar Gordian knots.

Brown v. Board of Education corrected historic errors and catalyzed change in the states, violent Southern resistance notwithstanding. It was a triumph.

Of course, one of the nation’s historic errors was the court’s own blunder in Dred Scott, which sought to unify the nation on the basis of slaveholder property rights — and succeeded only in dividing it beyond repair.

And then there is Roe v. Wade, which secured a woman’s right to choose abortion — but also stirred the pro-life movement, subjected the court to withering scholarly attack and forever politicized judicial nominations.

One of Roe’s strongest supporters on the court, Justice Ruth Bader Ginsburg, noted in remarks at Columbia University in February: “It’s not that the judgment was wrong, but it moved too far too fast.” At the time of Roe, abortion was legal in four states and legal under limited circumstances in about 16 others.

We’ll never know what would have happened if the court had let the democratic process play out in the matter of abortion.

What we do know is that gay marriage also confronts the unelected justices with delicate issues of morality, equal rights, federalism and democracy.

It does so at a time when democratic politics appear to offer advocates of same-sex marriage opportunities not available to, say, opponents of segregation at the time of Brown. Indeed, the political momentum behind marriage equality is so strong that its ultimate victory seems, if not inevitable, then nearly so. Even in the South, support for gay marriage is 14 percentage points higher than it was a decade ago, according to Pew. Political reality, in this case, may counsel judicial restraint.

Procedural and other particularities of the two cases before the court could permit the justices to keep a door open to gay marriage without slamming another on its opponents.

Marriage equality is important. So, too, are national harmony, self-government and the rule of law. Whatever it does, the Supreme Court should make it possible for the American people, sooner or later, to have them all.