OLYMPIA, Wash. -- Washington state has approved gay marriage, joining Maine and Maryland as the first states to pass same-sex marriage by popular vote.

Voter returns released since election night show Referendum 74 has maintained its lead of 52 percent. 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.

In Washington, more than half of the expected ballots have been counted and additional results are expected to be posted Thursday evening. Because Washington is a vote-by-mail state, and ballots only had to be postmarked by Tuesday, votes will continue to trickle in throughout the week.

The measure was losing in 31 of the state's 39 counties. But it had its strongest lead -- 66 percent of the vote -- in King County, which holds about a third of the state's voters and is home to Seattle.

Gregoire, who announced her support of gay marriage at the start of the legislative session in January, said she was proud of the result.

"Voters stood up for what is right and what is just and said that all Washington families are equal under the law," she said in a statement issued Thursday afternoon. "This is a day that historians will look back on as a turning point for equality."

Preserve Marriage Washington, meanwhile, issued a statement saying its members were disappointed but 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.