North Carolina voters approved a constitutional amendment on Tuesday defining marriage solely as a union between a man and a woman, making it the 30th state to adopt such a ban.
With 35 percent of precincts reporting Tuesday, unofficial returns showed the amendment passing with about 58 percent of the vote to 42 percent against.
In the final days before the vote, members of President Obama's Cabinet expressed support for gay marriage and former President Bill Clinton recorded phone messages urging voters to reject the amendment.
Meanwhile, supporters had run their own ad campaigns and church leaders urged Sunday congregations to vote for the amendment. The Rev. Billy Graham, who at 93 remains influential even though his last crusade was in 2005, was featured in full-page newspaper ads supporting the amendment.
Both sides spent a combined $3 million on their campaigns.
North Carolina law already bans gay marriage, like nine other states, but an amendment would effectively slam the door shut on same-sex marriages.
The amendment also goes beyond state law by voiding other types of domestic unions from carrying legal status, which opponents warn could disrupt protection orders for unmarried couples.
Six states -- most in the Northeast and including New York -- and the District of Columbia allow same-sex marriages.
The North Carolina amendment was placed on the ballot after Republicans took over control of the State Legislature after the 2010 elections, a role the GOP hadn't enjoyed for 140 years.
Joe Easterling, who described himself as a devout Christian, voted for the amendment at a polling place in Wake Forest.
"I know that some people may argue that the Bible may not necessarily be applicable, or it should not be applicable, on such policy matters. But even looking at nature itself, procreation is impossible without a man and a woman. And because of those things, I think it is important that the state of North Carolina's laws are compatible with the laws of nature but, more importantly, with the laws of God."
Linda Toanone, who voted against the amendment, said people are born gay and it is not their choice. "We think everybody should have the same rights as everyone else. If you're gay, lesbian, straight -- whatever," she said.
North Carolina is the latest presidential swing state to weigh in on gay marriage. Florida, Virginia and Ohio all have constitutional amendments against gay marriage, and Obama's election-year vagueness on gay marriage has come under fresh scrutiny.
Obama, who supports most gay rights, has stopped short of backing gay marriage.