North Carolina voters approved a constitutional ban on same-sex marriage Tuesday, dealing a setback to a gay rights movement that has enjoyed significant momentum in recent years.

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.

The national debate over gay marriage is turning its attention South. North Carolina voters pass a constitutional amendment defining marriage as solely between a man and a woman.

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.