Voters in North Carolina approve gay marriage ban.
The measure was passed by 61 percent against 39 percent as of 0230 GMT, according to preliminary results from the North Carolina State Board of Elections.
Similar state constitutional amendments have been approved in some 30 US states.
The amendment solidifies and expands already enacted North Carolina law forbidding same-sex marriage.
Money from national interest groups poured into North Carolina ahead of the election -- the National Organization for Marriage contributed $425,000 to the Vote for Marriage campaign, according to the latest reports, and the Human Rights Campaign contributed some $257,000 to the opposition, the Coalition to Protect All N.C. Families.
The Rev. Billy Graham, an evangelical preacher who was born and lives in North Carolina and at 93 remains enormously influential, took out full-page newspaper ads across the state supporting the ban.
"At 93, I never thought we would have to debate the definition of marriage," Graham said in the ads.
"The Bible is clear: God's definition of marriage is between a man and a woman."
On the other side, former president Bill Clinton and his ex-chief of staff, Erskine Bowles, a North Carolina native, recorded telephone messages to voters urging them to oppose the measure. In a statement, President Barack Obama's campaign also opposed the amendment.
"The passage of Amendment One is a profound injustice. Singling out a class of citizens for discriminatory treatment is unfair, unlawful and violates basic American values," said Adam Umhoefer with the American Foundation for Equal Rights, a leading US group supporting gay marriage, after the vote.