(CNN) -- North Carolina voters have passed a constitutional amendment banning same-sex marriage, CNN projects, putting a ban that already existed in state law into the state's charter.
With more than 2 million votes counted from Tuesday's referendum, supporters of the ban led opponents by a 61%-39% margin, according to figures from the State Board of Elections. Its backers celebrated by serving wedding cake to their supporters in a Raleigh ballroom.
Tami Fitzgerald, the head of Vote for Marriage NC, said she had been confident that "the people of North Carolina would rise up and vote to keep the opposition from redefining traditional marriage."
"We are not anti-gay, we are pro-marriage," she said. "And the point -- the whole point -- is simply that you don't rewrite the nature of God's design for marriage based on the demands of a group of adults."
The amendment alters North Carolina's constitution to say that "marriage between one man and one woman is the only domestic legal union that shall be valid or recognized in this state." Supporters argued that the amendment was needed to stop those trying to redefine marriage and ward off possible future actions of "activist judges."
Opponents called the measure redundant and warned it could result in jeopardizing domestic violence protections for women and affect couples' health benefits. Gov. Bev Perdue, a Democrat, also urged a no vote, arguing that writing a discriminatory provision into the state charter "is just plain wrong."
Jeremy Kennedy, campaign manager for the Coalition to Protect All North Carolina Families, acknowledged the loss and told opponents, "It's OK to grieve." But he urged advocates of same-sex marriage to keep fighting.
"All eyes in the nation are on us tonight, wondering not only how we came together to fight this epic campaign, but what is it going to leave behind -- what's going to happen to North Carolina after this," Kennedy said. "And I have to say that we are stronger for it. We are better for it. Our voices are louder now. We have courage like we've never had before."
"No" votes led in Charlotte, in the Triangle counties around Raleigh and in the Winston-Salem-Greensboro area, according to figures from the State Board of Elections. But it was winning by wide margins in rural counties and in the suburbs of Charlotte -- the home of famous evangelist Billy Graham, who endorsed the ballot measure last week.
Graham's endorsement was a rare move for a preacher who has typically avoided political fights. He took out full-page ads in 14 North Carolina newspapers touting his support for the measure, saying "the Bible is clear -- God's definition of marriage is between a man and a woman."
On the other side, former President Bill Clinton opposed the amendment in a recording sent by phone to hundreds of thousands of North Carolina homes.
"So the real effect of the law is not to keep the traditional definition of marriage, you've already done that," Clinton says in the recording. "The real effect of the law will be to hurt families and drive away jobs."
The amendment also would strengthen the state's position against same-sex civil unions, often considered a precursor to the marriage issue. Several municipalities in North Carolina provide benefits to same-sex couples, and Duke University law professor Kathryn Bradley said those rights could be lost with passage of the amendment.
Concerns over the measure also honed in on the potential for unintended consequences, she said, affecting issues such as child custody and the prosecution of domestic violence among unmarried couples because of the narrow definition of the new statute.
"Before domestic violence laws, we relied on criminal assault laws, which don't always protect against things like stalking," added Bradley, who says the measure could also affect heterosexual couples.
The state House and Senate voted in 2011 to put the amendment before state voters. Both chambers are Republican-controlled for the first time in the past 140 years.
Americans overall are closely split on the issue, according to a recent Gallup survey, with 50% of Americans believing same-sex couples should be allowed to wed -- up considerably from polls in past years. Another 48% say such marriages should not be legal.
Before Tuesday, 30 states had voted in favor of constitutional amendments that seek to defend traditional definitions of marriage as a heterosexual union.
Massachusetts, Connecticut, Iowa, Vermont, New Hampshire, New York and the District of Columbia issue marriage licenses to same-sex couples. In February, Washington Gov. Christine Gregoire signed a bill that would legalize same-sex marriage beginning in June, but opponents there have pledged to block the bill and called for voters to decide the issue.
Maryland Gov. Martin O'Malley signed into law a bill that permits same-sex couples to wed in that state as of January 1, 2013.
CNN's Joe Sutton, David Ariosto, Political Editor Paul Steinhauser and Belief Blog Co-Editor Eric Marrapodi contributed to this report.