The National Organization for Marriage (NOM) has vowed to repeal a proposed gay marriage law being debated in Washington state.

The Senate on Wednesday approved the measure with a larger than expected 28 to 21 vote. Supporters outnumber opponents in the House, making a vote in that chamber just a formality.

Governor Chris Gregoire for the first time publicly endorsed gay marriage last month, and promised to sign the bill if it reached her desk.

(Related: Chris Gregoire urges Americans to support gay marriage.)

NOM President Brian Brown called the move to legalize such unions “outrageous.”

“The same activists behind this legislation were barely able to secure passage of civil unions and now they're back demanding that marriage be redefined. It's outrageous,” Brown said in a statement. “NOM believes the people of Washington deserve to decide this issue for themselves, and we are committed to helping mount a referendum campaign to overturn the law if it is enacted, just as we did in Maine and California.”

Senator Ed Murray, the chief sponsor of the bill in the Senate, told the AP that he believes his side would win if the issue was put on the ballot.

“It will be a tough battle, it will probably be an ugly battle, but I think we'll win,” Murray said.

Opponents cannot begin a referendum campaign until after the bill is signed into law. They will need to collect 120,577 valid signatures by June 6 to put the issue on the November ballot.

According to an October survey, only 38 percent of respondents said they would reject a gay marriage law.

Washington currently recognizes gay and lesbian couples with domestic partnerships.