As lawmakers held their first public hearing on legalizing same-sex marriage, a previously undecided Democratic senator on Monday 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 announcement by Sen. Mary Margaret Haugen, D-Camano Island, that she would cast the 25th and deciding vote in favor of the issue came as hundreds of people filled the Capitol to advocate for and against gay marriage.

In a written statement issued at the end of a Senate committee hearing on the bill, Haugen said she took her time making up her mind to “to reconcile my religious beliefs with my beliefs as an American, as a legislator, and as a wife and mother who cannot deny to others the joys and benefits I enjoy.”

“This is the right vote and it is the vote I will cast when this measure comes to the floor,” she said.

The state House is widely expected to have enough support to pass gay marriage, and Gov. Chris Gregoire publicly endorsed the proposal earlier this month. 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 file a referendum to challenge it. Opponents have already said they will.

Democratic Sen. Ed Murray, a gay lawmaker from Seattle who has led the push for gay civil rights and domestic partnerships, testified before the Government Operations, Tribal Relations & Elections Committee with his longtime partner, Michael Shiosaki.

“I realize the issue of marriage for our families is emotional and divisive,” said Murray, who is sponsoring the Senate bill. “It touches what each of us holds most dear, our families.”

Others argued that the measure goes against traditional marriage and the Bible.

“You are saying as a committee and a Legislature that you know better than God,” said Ken Hutcherson, pastor of Antioch Bible Church.

The National Organization for Marriage, noting its involvement in ballot measures that overturned same-sex marriage in California and Maine, issued a statement Monday morning pledging a referendum campaign to fight any gay marriage law in Washington state.

“I want to re-emphasize that we fully expect that this issue is going to end up on the ballot,” said Rep. Jamie Pedersen, D-Seattle and sponsor of the House bill, said at a news conference following Haugen's announcement. “People should not be complacent.”

Gay marriage has won the backing of several prominent Pacific Northwest businesses, including Microsoft Corp. and NIKE, Inc., and last week a conservative Democrat who once opposed same-sex marriage said he will now vote for it.

Jane Abbot Lighty, 75, and her partner of 35 years, 84-year-old Pete-e Petersen, celebrated the vote-count announcement after the hearing.

“We could have gone out of state and gotten married,” said Lighty, of Seattle. “We want to be married in our home state.”

In October, a University of Washington poll found that an increasing number of people in the state support same-sex marriage. About 43 percent of respondents said they support gay marriage, up from 30 percent in the same poll five years earlier. Another 22 percent said they support giving identical rights to gay couples but just not calling it marriage.

When asked how they would vote if a referendum challenging a gay marriage law was on the ballot, 55 percent said they would vote yes to uphold the law, with 47 percent of them characterized as “strongly” yes, and 38 percent responded “no,” that they would vote to reject a gay marriage law.

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