SPOKANE, Wash. (RNS) Opponents of gay marriage promised a fight at the ballot box after the state Senate on Wednesday (Feb. 1) took a major step toward making Washington the seventh state to allow same-sex marriage.

After passing the Senate 28 to 21, the bill is now headed for expected approval in the House and on to Gov. Christine Gregoire, a Democrat, who has promised to sign it.

"There's still a lot of work to be done, we have to be diligent, but we're confident that this legislation will make it to the governor's desk," said Zach Silk of Washington United for Marriage, a statewide coalition fighting for marriage equality.

Opponents, however, are putting up a fight. They will have until July to collect more than 150,000 signatures to put the measure to a public vote on the November ballot.

"I am happy it passed, but it will undoubtedly face a referendum in the fall, so it's too soon to begin talking about what it all means," said Rev. Bill Ellis, dean of Spokane's Episcopal Cathedral of St. John the Evangelist.

The state's four Roman Catholic bishops are leading the fight against the bill. Seattle Archbishop J. Peter Sartain said redefining marriage insults the purpose and value of a sacred institution.

"Because only the union of a man and a woman can generate new life, no other human relationship is its equivalent," he said before the Senate vote.

Catholics are a religious minority in Washington, composing 16 percent of the state's population, according to the Pew Forum on Religion & Public Life. A quarter of the population identifies as evangelical, 23 percent are mainline Protestant and another 23 percent are not affiliated with any religion. 

Nationally, a majority of mainline Protestants (52 percent), Catholics (56 percent), and religiously unaffiliated (74 percent) favor marriage equality for same-sex couples, according to the Public Religion Research Institute.

"The (Catholic) leadership is in a very, very different place than the laity," said Ross Murray, director of faith and values for the New York-based Gay & Lesbian Alliance Against Defamation.

Historically when same-sex marriage legislation has gone to referendum, it's lost. "But 2012 might just be the year that changes," Murray said.

Currently gay marriage supporters are working to pass similar legislation in New Jersey and Maryland; voters in Maine will be asked whether to restore same-sex marriage after it was overturned in a 2009 referendum.