The campaign over same-sex marriage in Washington is about to shift into what is sure to be a contentious showdown over the next five months between supporters and opponents hoping to win over state voters.

Preserve Marriage Washington, the campaign opposed to gay marriage, on Wednesday submitted more than 241,000 signatures to the Secretary of State's Office, doubling the minimum of 120,577 needed to qualify its Referendum 74 for the November ballot.

It was the most signatures ever collected for a referendum in the state.

The group wants voters to reject the state's new same-sex marriage law, which the governor signed in February. It was to have taken effect Thursday, but will now remain on hold until voters approve it or throw it out.

State Sen. Ed Murray, D-Seattle, primary sponsor of the legislation, said he knew from the start this day would come.

"It's disappointing because this will divide the state," he said."From what we've seen elsewhere, it will be ugly and hurtful to a lot of families in Washington state."

Approval of Ref. 74 would allow gays to legally wed in Washington. And while rejection would repeal the law, Peter Nicolas, law professor at the University of Washington, said there's nothing preventing the Legislature from attempting to pass another bill.

Murray said he's prepared to do that if necessary: "I believe we will win, but barring some disaster, I will be back, year after year after year."

Over the next week, the Secretary of State's Election Division will clean up the more than 16,000 petition sheets the group submitted and randomly test the signatures for validity.

Both sides now are poised to take their messages directly to voters.

Zach Silk, campaign manager for Washington United for Marriage, said supporters of same-sex marriage, will "spend a lot of time having conversations in every corner of the state about how important it is that loving couples have the freedom to marry."

When asked, voters in 30 states have come out against same-sex marriage. And while Silk acknowledged that, he said, "we are confident we can win here."

He cited a broad coalition of support from rural and suburban lawmakers, from businesses and religious leaders and what he called "unique Washington values about freedom and fairness and treating people fairly."

And he pointed to polls that show the percentage rate of support in this state running in the mid-50s.

Washington, he said, has had a longer-running conversation with voters on same-sex unions than other states — including the Ref. 71 battle three years ago over the state's domestic-partnership law.

"We feel good going into the fall ... we also recognize that it's going to be a long, hard fight to November."

Joseph Backholm, campaign manager for Preserve Marriage, said his group's conversation with Washingtonians will center on "what marriage means, why it matters, how we see parenthood and family and our society structured over the next three or four generations, and the implications for education and religious freedom if we make marriage genderless."

He said gay-marriage supporters have been effective in convincing people that by "supporting the redefinition of marriage you prove you are a kind, decent person.

But, he said, "A big part of our job will be to convince people who know better that you can be a good and decent person without supporting a change in the definition of marriage."

Lornet Turnbull: 206-464-2420 or lturnbull@seattletimes.com.

On Twitter @turnbullL.