OLYMPIA — Washington state is on the verge of joining Maryland and Maine as the first states to approve gay marriage with a public vote.

With about half the expected ballots counted Tuesday night, Referendum 74 was passing with 52 percent of the vote. Counties were expected to post additional vote results this afternoon.

The measure 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.

Election parties in Seattle spilled out into the streets in the Capitol Hill neighborhood, where police closed off several blocks for an outdoor election celebration of President Barack Obama’s re-election, and where more than 1,000 people were dancing and chanting “74, 74, 74.”

The measure was losing in 31 of the state’s 39 counties. But it had its strongest lead — 65 percent of the vote — in King County, the state’s largest county and home to Seattle.

State Sen. Ed Murray, a Democratic gay lawmaker from Seattle who sponsored the marriage law that passed the Legislature, said he felt confident that Washington state’s numbers would hold.

“We’re almost there, and we should celebrate,” he said.

About $13.6 million was spent on the campaign 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 compared to the $2.7 million raised by Preserve Marriage Washington, which opposes the law.

“We remain cautiously optimistic that when all the ballots are in that the voters of Washington will not redefine marriage,” said Chip White, a Preserve Marriage spokesman. “We don’t need to win King County to win the state.”