Correction: An earlier version of this story erroneously reported that same-sex marriage bans would be on the ballot in Maryland and Washington state. Voters in those states are being asked to affirm measures that would allow gay marriage. The story has been corrected.
With less than a third of the returns tallied, the measure had enough support to pass, according to the Associated Press. It strengthens a same-sex marriage ban already on the books in North Carolina, which until Tuesday had been the only state in the Southeast that had not taken the step of incorporating the ban into its constitution.
Opponents of same-sex marriage said the measure was necessary to prevent courts or future legislatures from invalidating the law. The effort provoked an outcry from gay rights groups, which said it represented a significant step backward because it would bar even civil unions and could have unintended consequences for heterosexual couples.
The debate drew scrutiny from around the country, with the Obama campaign and former president Bill Clinton voicing their opposition to the measure and figures such as the Rev. Billy Graham and former presidential hopeful Newt Gingrich speaking out in favor of it.
The vote came as Obama is under pressure to clarify his position on same-sex marriage, a hot-button issue that has proved to be a galvanizing force for conservatives and liberals alike.
It also came amid a sea change of public opinion on same-sex unions. Polls show a slight majority of Americans support same-sex marriage, a dramatic shift from just eight years ago. Six states and the District allow gay couples to wed, and gay rights groups have accumulated a string of legal victories.
The gains have provoked a backlash from opponents, who argue that heterosexual marriage is a building block of society because of its role in procreation. They believe they have ceded too much ground and view the North Carolina constitutional amendment as a significant win because it is so broad.
On Tuesday, socially conservative groups pointed to the vote as evidence that the American public remains opposed to same-sex marriage, despite the polls.
“This overwhelming support for marriage is clearly the reason why President Obama and liberal congressional candidates across the country have not expressed open support for same-sex marriage,” Tony Perkins, president of the Family Research Council, said in a statement. “They know that redefining marriage remains a losing position in mainstream American politics.”
Gay rights groups late Tuesday lamented their loss but called it a temporary setback.
“The passage of Amendment One is a heartbreaking loss for families in North Carolina, but will not stop us in the march toward full equality,” Joe Solomonese, president of the pro-gay-rights Human Rights Campaign, said in a statement.
Though turnout was moderate Tuesday, a record 500,000 people cast early ballots, which elections officials attributed to passion over the same-sex marriage ban. Polls showed solid support for the measure among likely voters, including African Americans, a key Democratic constituency that has remained skeptical about gay marriage.
Supporters of same-sex marriage argued that the amendment could have a ripple effect, complicating domestic violence cases and limiting the rights of unmarried heterosexual couples. They cited analyses by legal scholars who pointed to Ohio, where a similar measure temporarily wreaked havoc in the courts. But other legal experts, whose work was highlighted by gay-marriage opponents, have said there was little evidence to support that claim.
Conservatives in North Carolina had long sought to put a constitutional ban on same-sex marriage on the ballot. Year after year they were thwarted by Democrats in the state legislature. But in 2010, Republicans swept both houses for the first time since 1870, giving conservatives the opportunity to move forward on the issue.
Despite the gains by supporters of same-sex marriage, they have never won a referendum. Gay rights groups hope to break that streak in November.
Same-sex marriage will be on the ballot in Maryland, Minnesota, Washington state and Maine.