"The numbers point to victory," said Zach Silk, a spokesman for Washington United for Marriage. "We're really feeling good."
R-74 asked Washingtonians to approve or reject a state law legalizing same-sex marriage that lawmakers passed earlier this year. That law was signed by Gov. Chris Gregoire but has been on hold pending the election's outcome.
With about half of the expected ballots counted Tuesday night, R-74 was passing with 52 percent of the vote. Counties were expected to post additional results Wednesday afternoon. Because Washington is a vote-by-mail state, and ballots only needed to be postmarked by Tuesday, votes will continue to trickle in throughout the week.
Silk said he was confident supporters would hold or build on their current lead and that Washington state would join Maryland and Maine, which both approved gay marriage measures Tuesday night. In Minnesota, voters rejected a proposed constitutional amendment to ban gay marriage in their state. Gay marriage remains illegal under Minnesota state law.
In Washington state, many supporters started celebrating Tuesday night, taking to the streets in a Seattle neighborhood and cheering at election watch parties as early results showed the referendum taking a narrow lead. Police closed off several blocks in Seattle's Capitol Hill area late Tuesday as more than 1,000 people gathered for an impromptu election celebration, dancing and chanting "74, 74, 74."
At a news conference Wednesday afternoon, gay marriage supporters crowded a room in Seattle to celebrate the vote count. Rep. Laurie Jinkins, D-Tacoma, was joined by her partner of nearly 24 years, Laura Wulf, and their 12-year-old son, Wulf.
"I can't wait to marry her," Jinkins said, before leaning down to kiss Laura Wulf. "I want to thank the voters of the state of Washington for allowing Laura and I to be married. It's a great day."
Even as the local campaign said it's waiting for more ballots to be counted, officials with the National Organization for Marriage noted they were "very disappointed in losing four tough election battles by narrow margins."
"We knew long ago that we faced a difficult political landscape with the four marriage battles occurring in four of the deepest-blue states in America," the group's president, Brian Brown, wrote in a news release issued Wednesday.
Preserve Marriage Washington, which also opposes the law, was waiting to see additional numbers from the counties, spokesman Chip White said.
"While we understand that the math is difficult, there is still a path to victory for our side," he said. "Everyone needs to respect the process and wait for the votes to be counted."
About $13.6 million was spent on the campaign, with the bulk of it coming from gay marriage supporters. Washington United for Marriage far outraised its opponents, bringing in more than $12 million compared with the $2.7 million raised by Preserve Marriage Washington.
If Referendum 74 does pass, gay couples 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 state 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.
Six other states - New York, Connecticut, Iowa, Massachusetts, New Hampshire, Vermont - and the District of Columbia already allow marriage. Maryland and Maine were the first to enact the law by public vote. In the other states, the laws were enacted either by lawmakers or through court rulings.