OLYMPIA, Wash. (AP) - Washington state voters on Tuesday were narrowly approving the state's new "everything but marriage" law that marks a significant expansion of rights for gay couples.

With about 50 percent of the expected vote counted, Referendum 71 was leading 51 percent to 49 percent.

The measure asked voters to approve or reject the final expansion to the state's domestic partnership law, which grants registered domestic partners additional state-granted rights currently given only to married couples.

Supporters are hoping their narrow lead holds as votes continue to be counted. Elections officials are expected to release additional results Wednesday afternoon.

There was an east-west divide to the results. It was passing in 10 counties in traditionally more liberal western Washington, including King County, which includes Seattle and holds about a third of the state's voters. Eastern Washington counties were overwhelmingly voting to reject, as were counties in southwest Washington.

"This is a remarkable statement by the voters of Washington," said Anne Levinson, chairwoman of Washington Families Standing Together, the group fighting to keep the law on the books. "They have a history of being fair-minded and compassionate, and they've proven that once again."

Levinson said that if the vote trend continues, "we'll be making history in our state in moving forward toward full equality for gay and lesbian families in our state."

The expanded law would add benefits, such as the right to use sick leave to care for a domestic partner, and rights related to adoption, child custody and child support.

Opponents remained optimistic that they could close the gap, but warned that if the law was approved, gay marriage would be the next step in Washington state.

"I think it will be immediate," said Gary Randall of Protect Marriage Washington, which pushed to get the referendum on the ballot. "Now was the time to stand against it."

The law was supposed to take effect July 26, but now will go on the books only if approved by voters. If rejected, previously enacted legislation on domestic partnerships with fewer benefits to gay couples would remain in place.

The underlying domestic partnership law, which the Legislature passed in 2007, provided hospital visitation rights, the ability to authorize autopsies and organ donations, and inheritance rights when there is no will. Under state law, senior couples can register as domestic partnerships as well.

Last year, lawmakers expanded that law to give domestic partners standing under laws covering probate and trusts, community property and guardianship.

More than 12,000 people in Washington state are registered as domestic partners.

Washington state, along with several other states including California, Oregon and New Jersey, have laws that either recognize civil unions or domestic partnerships that afford same-sex couples similar rights to marriage.

Same-sex marriage is legal in Massachusetts, Connecticut, Iowa and Vermont, and will start in New Hampshire in January. Voters in Maine on Tuesday repealed a gay marriage law that was passed by the Legislature there earlier this year.

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County-by-county breakdown of R-71 vote