OLYMPIA, Wash. — A measure to legalize same-sex marriage in Washington state moved one step closer to passage on Thursday as a Senate committee voted to approve it.

The Government Operations, Tribal Relations & Elections Committee approved the measure with a 4-3 voice vote split on party lines. The bill is expected to head to a floor vote next, though a date has not yet been determined.

Several Republican amendments were rejected, including one that would have required that the measure be sent to the November ballot.

"This is a significant change in law," said Sen. Don Benton, R-Vancouver. "It will change our society in ways that nobody here can even foresee 30, 40, 50 years from now. Something that significant, I strongly believe, should be placed before the citizens for their approval."

Committee chairman, Democratic Sen. Craig Pridemore of Vancouver, said the state should no longer discriminate against gay and lesbian couples who want to get married.

"In my opinion, this bill rights a longstanding wrong," he said after the vote.

Opponents of same-sex marriage have already promised a referendum battle at the ballot if the Legislature passes the bill and it's signed into law.

Washington state has had a domestic partnership law since 2007, and an "everything but marriage" expansion of the domestic partnership law since 2009.

Before this week, it wasn't certain the Senate would have the support to pass the measure, due to a handful of undecided Democrats. But on Monday, after the first public hearing on the issue, a previously undecided Democratic senator, Mary Margaret Haugen of Camano Island announced her support for the measure, all but ensuring that Washington will become the seventh state to allow gay and lesbian couples to get married.

The state House already had secured enough votes to pass the measure, and Democratic Gov. Chris Gregoire publicly endorsed the proposal earlier this month.

Bills have been introduced in both the House and the Senate, both sponsored by gay lawmakers who have been active in promoting gay rights issues before the Legislature. The House Judiciary Committee is scheduled to vote on its companion bill on Monday.

The gay marriage bills have the backing of several prominent Pacific Northwest businesses, including Microsoft Corp., NIKE Inc. and Starbucks.

If a marriage bill is passed during this legislative session, gay and lesbian couples will be able to get married starting in June unless opponents follow through on their threat to file a referendum to challenge it.

A referendum can't be filed until after the bill is passed by the Legislature and signed into law by Gregoire. Opponents then must turn in 120,577 signatures by July 6.

Same-sex marriage is legal in New York, Connecticut, Iowa, Massachusetts, New Hampshire, Vermont and the District of Columbia. Lawmakers in New Jersey and Maryland are expected to debate gay marriage this year, and Maine could see a gay marriage proposal on the November ballot. On Thursday, gay rights activists there announced they had collected enough signatures to seek a referendum. The Maine Legislature previously approved gay marriage, but it was rejected by a 2009 statewide vote, 53 percent to 47 percent.

The gay marriage bills are Senate Bill 6239 and House Bill 2516.