"With added results showing that we have not closed the gap, it now appears clear that Referendum 74 will be narrowly approved," Preserve Marriage Washington Chairman Joseph Backholm said. "We are disappointed in losing a tough election battle on marriage by a narrow margin. But while we are disappointed, we are not defeated."
Latest results showed the measure passing by 52 percent to 48 percent. Referendum 74 allows clergy and churches to refuse to perform same-sex marriage ceremonies or recognize the unions.
The Evergreen State is the ninth state to approve same-sex marriage and the third to do so in 2012.
Zach Silk, campaign manager for Washington United for Marriage, said, "This is a historic day for Washington, a historic day for our country and, most of all, for families across the state who have dreamed of this day."
The race for governor was still too close to call, with Democrat Jay Inslee holding a slightly reduced lead -- about 51 percent to 49 percent -- over Republican Rob McKenna. The victor will replace outgoing Democratic Gov. Christine Gregoire.
McKenna told supporters Thursday ballots mailed over the past week will "carry me to victory," the Seattle Post-Intelligencer reported.
Inslee was ahead by 48,776 votes out of more than 2 million, the newspaper said.
Northwest Public Radio reported Inslee said Tuesday night he thought his lead would hold up.
"I believe tonight our state has taken another step forward and we have elected a forward looking governor and I fully intend to be that governor for the state of Washington," Inslee said.
McKenna said Tuesday he wasn't ready to concede and asked his supporters to hold on, possibly a few more days, Northwest Public Radio said.
"We are confident that thanks to your hard work and dedication that when the last ballots are counted it will be clear that voters have sent a clear message that it is time to move our state in a new direction," McKenna said.
Cantwell cruised past Republican challenger Michael Baumgartner with 59 percent of the vote.
The Democrats captured six of the 10 House seats up for grabs. The GOP won the other four.
A ballot measure decriminalizing marijuana possession and regulating its cultivation passed Tuesday with 55 percent of the vote and 51 percent of the ballots counted.