The state Senate at Washington now has enough supporters of equal marriage rights to pass draft legislation.
Mary Margaret Haugen, a Democratic state senator representing Camano Island became the 25th senator to back the move to allow gay marriage.
This creates a majority in the state senate, where 49 members sit, and the House of Representatives is expected to pass the legislation.
But opponents may still force a state-wide ballot in July if the legislation is successful.
In a statement, Senator Haugen said: “I know this announcement makes me the so-called 25th vote, the vote that ensures passage. That’s neither here nor there. If I were the first or the seventh or the 28th vote, my position would not be any different.
“I happen to be the 25th because I insisted on taking this much time to hear from my constituents and to sort it out for myself, 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.”
In her full statement, the state senator said the issue was one of living alongside other Americans, writing: “For as long as I have been alive, living in my country has been about having the freedom to live according to our own personal and religious beliefs, and having people respect that freedom.”
On her decision to vote in favour of marriage equality for gay couples, she wrote: “Do I not feel they should have the right to do as they want? My beliefs dictate who I am and how I live, but I don’t see where my believing marriage is between a man and a woman gives me the right to decide that for everyone else.”
Washington would become the seventh in the US to lift the ban on gay marriage.
Haugen has been a Congresswoman since 1982, serving in the state House of Representatives before moving to the Senate in 1993.
In order to force a ballot on the issue, the law must be passed and 120,577 signatures must be collected by 6 July.
A pastor from the Antioch Bible Church quoted by the Washington Post said the legislature was effectively saying it “knew better than God”.
Lacey All, Chair of the pro-equality coalition Washington United for Marriage, said: “Washington United commends and congratulates Sen. Haugen for her support of marriage equality.
“We’ve known for a long time that our stories are powerful, and sharing those stories can change hearts and minds. Hundreds of constituents shared their stories of love, commitment and family with Sen. Haugen, and in doing so convinced her that she was doing the right thing for Washington.”
Stuart Gaffney, Marriage Equality USA Media Director, said: “Fair-minded Americans everywhere are celebrating with Washington State today, as Senator Haugen’s announcement ensures passage of equal marriage rights in Washington. 2012 is already shaping up to be another great year for loving, committed couples from coast to coast.”
“New York’s leadership in 2011 is now followed by Washington State’s leadership in 2012,” added John Lewis, Marriage Equality USA Legal Director. “The trend nationwide couldn’t be clearer, and we look forward to state after state deciding to treat all marriages equally.”
An initial senate committee vote on the bill SB 6329 will be held on Thursday, which the bill is expected to pass easily.