OLYMPIA, Wash. - The Washington state Senate on Wednesday passed a bill that would legalize same-sex marriage, setting the stage for the state to become the seventh to allow gay and lesbian couples to wed.

The measure now heads to the House, which is expected to approve it. Gov. Chris Gregoire supports the measure and has said she will sign it into law, though opponents have promised to challenge it at the ballot with a referendum.

The packed public galleries burst into applause as the Senate passed the measure on a 28-21 vote Wednesday night after nearly an hour and a half of debate. Four Republicans crossed party lines and voted with majority Democrats for the measure. Three Democrats voted against it.

Democratic Sen. Ed Murray, the bill's sponsor, said he knew same-sex marriage "is as contentious as any issue that this body has considered in its history."

Lawmakers who vote against gay marriage "are not, nor should they be accused of bigotry" he said.

"Those of us who support this legislation are not, and we should not be accused of, undermining family life or religious freedom," said Murray, a gay lawmaker from Seattle who has spearheaded past gay rights and domestic partnership laws in the state. "Marriage is how society says you are a family."

Nearly a dozen amendments were introduced, including several that passed that strengthen legal protections for religious groups and organizations.

Sen. Dan Swecker, R-Rochester argued that the proposed law alters the definition of marriage and "will lead to the silencing of those who believe in traditional marriage."

Even though a referendum clause amendment was rejected, opponents have already promised to file a challenge, which can't be done until after it is passed by the full Legislature and signed into law by Gregoire. Opponents then must turn in 120,577 signatures by June 6.

If opponents aren't able to collect enough signatures, gay and lesbian couples would be able to be wed starting in June. Otherwise, they would have to wait until the results of a November election.

Before last week, it wasn't certain the Senate would have the support to pass the measure, as a handful of Democrats remained undecided.

But after the first public hearing on the issue Jan. 23, a previously undecided Democratic senator, Mary Margaret Haugen of Camano Island, said she would be the 25th and deciding vote in support of the bill, all but ensuring its passage.

Gay marriage opponent Jane Sterland, 56, stood outside the Senate gallery before the debate started. Sterland said she was disappointed by the light turnout of same-sex marriage foes.

"It saddens me that there aren't more Christians here tonight," she said. "I'm just very grieved about this whole thing. I want to be here for prayer support against this issue."

Alex Guenser, a 26-year-old engineer, drove down to Olympia from his Redmond home with his boyfriend to watch the Senate debate.

"I feel like this is the hill, the crest of the marriage equality fight. And after this passes (in the Senate), it's all going to be smoother sailing from now on," Guenser said. "I'm really excited to have Washington pass this. I'm excited for my state."

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.

The debate over same-sex marriage in Washington state has changed significantly since lawmakers passed Washington's Defense of Marriage Act in 1998, which banned gay marriage. The constitutionality of DOMA was ultimately upheld by the state Supreme Court in 2006, but earlier that year, a gay civil rights measure passed after nearly 30 years of failure.

The quick progression of domestic partnership laws in the state came soon after, with a domestic partnership law in 2007, and two years of expansion that culminated in 2009 with the so-called "everything but marriage law" that was upheld by voters after opponents filed a referendum to challenge it.

Under the measure that passed Wednesday, the more than 9,300 couples currently registered in domestic partnerships would have two years to either dissolve their relationship or get married. Domestic partnerships that aren't ended prior to June 30, 2014, would automatically become marriages.

Domestic partnerships would remain for senior couples where at least one partner is 62 years old or older. That provision was included to help seniors who don't remarry out of fear they could lose certain pension or Social Security benefits.

The gay marriage bill is Senate Bill 6239.