Voter returns released since election night show Referendum 74 has maintained its lead. Opponents conceded the race Thursday, while supporters declared victory a day earlier.
Zach Silk, a spokesman for Washington United for Marriage, called it a "historic day."
"We have always understood that there are good people on the other side of this issue," he said in a statement issued Thursday. "Yet, we remain confident that once people see how much marriage matters to families, they will realize that the love and commitment that marriage embodies only strengthens families, neighborhoods and communities."
R-74 asked people to approve or reject a state law legalizing same-sex marriage that legislators passed earlier this year. That law was signed by Gov. Chris Gregoire but has never taken effect. It was on hold pending the election's outcome.
Washington is one of four states where voters were asked about the issue this election cycle. Maryland and Maine approved gay marriage Tuesday night, while Minnesota voters rejected a proposed constitutional amendment to ban same-sex marriage.
Six other states — New York, Connecticut, Iowa, Massachusetts, New Hampshire, Vermont — and the District of Columbia already allowed gay marriage. But Maryland, Maine and Washington are the first to enact it by public vote. The other states' laws were enacted either by lawmakers or court rulings.
Preserve Marriage Washington issued a statement Thursday saying that while its members were disappointed with the results, they "will continue to educate citizens and policymakers on the timeless truth that real marriage is the union of one man and one woman."
"We are disappointed in losing a tough election battle on marriage by a narrow margin," said Joseph Backholm, the campaign chairman.
Backholm blamed several factors, saying Washington is a "deep blue state."
"The election results reflect the political and funding advantages our opponents enjoyed in this very liberal and secular state," he wrote. "The results show only that in a deep blue state, with a huge financial advantage, gay marriage activists can win — barely."
About $13.6 million was spent on the initiative in Washington state, with the bulk of it coming from gay marriage supporters. Washington United for Marriage far outraised its opponents, bringing in more than $12 million, including donations from big names like Amazon.com CEO Jeff Bezos, Microsoft Corp. co-founder Bill Gates and New York Mayor Michael Bloomberg. Opponents of gay marriage raised just $2.7 million.
Many supporters started celebrating early, taking to the streets in a Seattle neighborhood and cheering at election watch parties Tuesday night as early results showed the referendum taking a narrow lead. Police closed off several blocks in Seattle's Capitol Hill area as more than 1,000 people gathered for a late-night, impromptu election celebration, dancing and chanting "74, 74, 74."
Gay couples in Washington could start picking up their marriage certificates and licenses from county auditor offices Dec. 6, a day after the election is certified. However, because Washington has a three-day waiting period, the earliest a certificate could be signed, making the marriage valid, is Dec. 9.
The law doesn't require religious organizations or churches to perform marriages, and it doesn't subject churches to penalties if they don't marry gay or lesbian couples.
The road to gay marriage in Washington state began several years ago.
A year after Washington's gay marriage ban was upheld by the state Supreme Court, the state's first domestic partnership law passed in 2007. That law granted couples about two dozen rights, including hospital visitation and inheritance rights when there is no will. It was expanded a year later, and then again in 2009, when lawmakers completed the package with the so-called "everything but marriage" bill. Voters upheld the bill later that year.
This year, lawmakers passed the law allowing gay marriage, and Gregoire signed it in February. Preserve Marriage gathered enough signatures for a referendum, putting the law on hold before it could take effect.