Supporters of the amendment responded with marches, television ads and speeches. Church leaders urged Sunday congregations to vote for the amendment. The Rev. Billy Graham was featured in full-page newspaper ads backing the amendment.
North Carolina law already bans gay marriage, but an amendment effectively seals the door 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.
The campaign manager for the group that opposed the amendment said the nation watched North Carolina on Tuesday night, wondering how the anti-forces came through.
"I am happy to say that we are stronger for it; we are better for it; our voices are louder now," said Jeremy Kennedy of Protect All NC Families. "We have courage like we never had before, and we have strength to continue on."
Both sides spent a combined $3 million on their campaigns.
Six states - all in the Northeast except Iowa - and the District of Columbia allow same sex marriages. In addition, two other states have laws that are not yet in effect and may be subject to referendums
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 Republican Party 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. Without clarification, he's said for the past year-and-a-half that his personal views on the matter are "evolving."
Later Tuesday, President Obama's campaign said he was "disappointed" with the amendment. Obama campaign spokesman Cameron French said in a Tuesday statement that the ban on same-sex unions is "divisive and discriminatory." Same-sex couples deserve the same rights and legal protections as straight couples, said French.
Education Secretary Arne Duncan stated on Monday his unequivocal support for same-sex marriage, a day after Vice President Joe Biden said he is "absolutely comfortable" with same-sex married couples getting the same rights at heterosexual married couples.
One fault line that could determine the result is generational. Older voters, who tend to be more reliable voters, were expected to back the amendment.
State House Speaker Thom Tillis, a Republican from a Charlotte suburb, said earlier in the year that even if the amendment passed, it would be reversed as today's young adults age - within 20 years. "It's a generational issue," Tillis told a student group at North Carolina State University in March about the amendment he supports.
"Also, that amendment is against women, I believe, because also underneath the amendment, other laws are saying that people who aren't married at all, they can't file for domestic abuse cases, if they're living with their significant other. Which is wrong," said Toanone.
In North Carolina, more than 500,000 voters had cast their ballot before Tuesday, which was more than the 2008 primary when Obama and Hillary Clinton were fighting for the Democratic presidential nomination. Both sides said that bodes well for them.
Intellpuke: You can read this article by Associated Press writer Martha Waggoner, reporting from Raleigh, North Carolina, in context here: www.guardian.co.uk/world/feedarticle/10233289
Associated Press writers Allen Reed, Allen G. Breed, Emery P. Dalesio and Gary D. Robertson contributed to this report.