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

With about three-quarters of the expected ballots counted Thursday, Referendum 74 was maintaining 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 approve it by public vote. The other states' laws were enacted either by lawmakers or court rulings.

In Washington, additional voter returns will continue to be posted daily. Because Washington is a vote-by-mail state, and ballots only had to be postmarked by Tuesday, votes will continue to trickle in for days.

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.

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

"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 disheartened 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.

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 rights supporters in neighboring Oregon said the passage of same-sex marriage measures in other states gives them hope for 2014.

Domestic partnerships are legal in Oregon, but same-sex marriage was banned by ballot measure in 2004.

Jeana Frazzini, director of the gay-rights group Basic Rights Oregon, said they considered going to the ballot to overturn the constitutional amendment this year but abandoned the effort in part because there wasn't clear evidence it could pass.

"Basic Rights Oregon is committed to leading the way to make Oregon the first state to overturn a constitutional amendment banning marriage," she said in statement.

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 path to gay marriage in Washington state began several years ago.

A 2006 state Supreme Court ruling upheld a 1998 law banning same-sex marriage. But earlier that year, a gay civil rights measure passed after the idea had failed for nearly 30 years, signaling a change of mindset in the Legislature.

The state's first domestic partnership law passed in 2007, granting 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" law that was ultimately upheld by voters 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.

State Sen. Ed Murray, a Democratic gay lawmaker from Seattle, sponsored the law. He said the election's outcome "rectifies an injustice that was done to gay and lesbian couples" when the state Legislature passed the Defense of Marriage Act banning gay marriage in the state.

Murray was in his third year as a state representative when that 1998 law passed. At the time, he was the Legislature's lone gay lawmaker.

"I believe that when the marriages start happening next month, that will be a healing moment for this state," he said.

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AP writer Jonathan J. Cooper contributed from Portland, Ore. Follow Rachel La Corte at http://www.twitter.com/RachelAPOly or http://www.facebook.com/news.rachel