( CNN ) -- Thousands of North Carolina voters hit the polls on Tuesday to cast their ballots on a referendum that would constitutionally ban same-sex marriage , even though state law already does not permit such unions . Amendment 1 would alter the North Carolina 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 argue that the amendment is needed to stop those trying to redefine marriage and to ward off possible future actions of `` activist judges . '' It would also strengthen the state 's position against same-sex civil unions , often considered a precursor to the marriage issue . Opponents say the measure is redundant and could result in jeopardizing domestic violence protections for women and affect couples ' health benefits . Follow all the political news at CNN.com 's Election Center . Several municipalities in North Carolina provide benefits to same-sex couples , state officials say . Duke University law professor Kathryn Bradley says those rights could be potentially lost if the referendum passes on Tuesday . Concerns over the measure also home 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 do n't always protect against things like stalking , '' added Bradley , who says the measure could also affect heterosexual couples . But Sen. Dan Soucek , one of the primary sponsors of the proposed amendment , said he does n't `` believe those are legitimate concerns . '' `` We looked at these issues extensively and tried to make sure that all of those concerns are addressed , '' he said . A simple majority is needed for the measure to be approved . It was passed in the state House and Senate last year . Both are Republican-controlled , the first time that 's happened in North Carolina in the past 140 years , according to house officials . `` This bill 's been 12 years in the making , '' noted Maxine Eichner , a law professor at University of North Carolina at Chapel Hill . `` But it 's now made possible because Republicans control the legislature . '' Should Amendment 1 gain approval , it would largely prevent the state 's judiciary from overturning the bill by enshrining its language into the state constitution . Some 500,000 people had already cast ballots on the measure through early voting or absentee ballots . Polls are set to close at 7:30 p.m . ET . `` It could destroy our family unit , '' said Kelli Evans alongside and her partner of 17 years , Karen Wade . `` If they could see that it 's a family they 're affecting , maybe they could sit down and think about the issue before they vote on Election Day . '' Others say the issue , if left unchecked , could one day undermine traditional notions of marriage . Evangelist Billy Graham endorsed the ballot initiative , a rare move for a preacher who has typically avoided political fights . Graham 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 . '' Graham 's website encouraged churches to download a poster that bears his image and the message `` Vote for Marriage May 8th . '' 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 only recent public opinion polling on the issue , derived from a group that does work for Democratic candidates and causes , indicates that a majority of North Carolina voters support the amendment . Nationally , according to a new Gallup survey , 50 % of Americans believe same-sex couples should be allowed to wed , a markedly different position than in past years , suggesting a growing acceptance of same-sex marriages . Some 48 % say they do not support same-sex weddings . Sen. Soucek described marriage on Tuesday as a `` time-tested building block of society that revolves around procreation . '' But three days earlier , Vice President Joe Biden said he was `` absolutely comfortable '' with the idea of same-sex marriage . `` I just think that the good news is that as more and more Americans come to understand what this is all about , it is a simple proposition : Who do you love ? Who do you love ? And will you be loyal to the person who love ? '' Biden said on NBC 's `` Meet the Press . '' Biden did not mention the North Carolina initiative and added that it 's the president , not he , who sets the administration 's policy . President Barack Obama has taken the official position that his views on the issue are `` evolving . '' Thirty states have voted in favor of constitutional amendments that seek to defend traditional definitions of marriage as a hetereosexual union . `` Of states without constitutional amendments on marriage , 45 % ( 9 of 20 ) eventually recognize same-sex marriage , either by direct judicial decree , by legislative action , or by a ruling requiring that same-sex marriages from other states be treated as valid , '' a nonprofit political group called the National Organization for Marriage said in a statement . `` Among the 30 states with marriage amendments , none have been repealed . '' 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 . Opponents there have pledged to block the bill and have also called for a referendum . 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 David Ariosto , Political Editor Paul Steinhauser and Belief Blog Co-Editor Eric Marrapodi contributed to this report .
Polls in North Carolina close at 7:30 pm . Tuesday . Both chambers of N.C. legislature are Republican - controlled for first time in past 20 years . Indiana and West Virginia also hold elections on Tuesday .