WILMINGTON, N.C. (RNS) As the only Southern state without a constitutional amendment barring same-sex marriage, North Carolina is the next battleground, with religious groups on both sides bracing for a high-stakes fight on May 8.

Against a recent string of gay-marriage victories in California, Washington state and Maryland, North Carolinians will be asked to vote on a constitutional amendment on May 8, the same day as the state Republican primary.

Same-sex marriage has been illegal in the Tar Heel State since 1996;  Minnesota also has a marriage amendment planned for a vote in November.

"Marriage between one man and one woman is the only domestic legal union that shall be valid or recognized in this state," the proposed amendment reads.

"This section does not prohibit a private party from entering into contracts with another private party; nor does this section prohibit courts from adjudicating the rights of private parties pursuant to such contracts."

Faith groups that support the amendment make a biblical argument for marriage between one man and one woman and say that without a permanent amendment, judges could overturn the state law that prohibits same-sex marriages.

Religious groups that oppose the amendment say same-sex marriage is about civil rights, not religion, and they oppose placing language into the state's constitution that discriminates against a minority.

A January poll by the Raleigh-based Public Policy Polling found that 56 percent of respondents supported the amendment, while 36 percent would vote against it. Ten percent were undecided.

The state's two top Catholic leaders, Bishops Peter Jugis of Charlotte and Michael Burbidge of Raleigh, support the amendment.

"We ask you, no matter what position you currently hold about the definition of marriage, to become informed about our church's teaching, to be respectful in all discussions about this important issue and to vote," the bishops wrote in a Jan. 29 joint letter to the state's Catholics.

"We pray that you will see our position as a principled one based on eternal and divine truth and that you will vote on May 8 for protecting traditional marriage in North Carolina."

Many conservative Christian groups in the state are gathering behind the NC Values Coalition and Vote for Marriage NC.

Tami Fitzgerald, the leader of Vote for Marriage NC, said California should be a wake-up call for North Carolina. A federal appeals court recently overturned the state's constitutional ban on same-sex marriage, in part because gay couples had the right to wed before voters banned the practice under 2008's Proposition 8.

"Every day that goes by is another day when a judge can decide to substitute his values for those of North Carolinians," she said. "We need the marriage protection amendment to prevent that."

On the other side of the issue, the Episcopal Diocese of North Carolina's annual convention recently approved a resolution opposing the amendment.

The North Carolina Council of Churches usually stays neutral on political arguments that divide its membership, but opposes the amendment, said executive director George Reed.

The state's two Catholic dioceses are members of the ecumenical group.

"We have a policy statement ... that this is discriminatory, and it's inappropriate to put language like that into our state's constitution," Reed said. "I have great respect for the dioceses, and I know they come to their position by their faith."

Though some black churches have joined the Vote for Marriage NC effort, the North Carolina NAACP, which includes thousands of African-American pastors statewide, decided to oppose the amendment.

Black Christians, who are among the religious groups most opposed to homosexuality, make up 13 percent of North Carolinians, according to the Pew Forum on Religion & Public Life, higher than the national average of 7 percent.

Some nonreligious lobbying groups, such as Protect All NC Families, have their own faith campaigns to coordinate church support, offering congregational get-out-the-vote tool kits. Faith outreach director Ryan Rowe said the amendment could have unintended consequences for benefits and domestic violence protections for heterosexual unmarried couples.

The group argues the amendment could harm many unmarried heterosexual couples as well. According to the 2010 U.S. Census, North Carolina had about 222,000 unmarried couples; only 22,000 of those were identified as gay or lesbian.

"No God I know would condone hurting people like this," Rowe said. "When you restrict the definition of a domestic legal union, that harms the children and partners of unmarried families."