UPDATE: Officials from Washington United for Marriage (WUM) declared a 52 to 48 percent win for Referendum 74 on Nov. 7.
"This is an historic day for Washington, an historic day for our country and, most of all, for families across the state who have dreamed of this day and the wedding celebrations to come," WUM campaign manager Zach Silk said in a statement on the group's website.
Given the state's mail-in voting system, Washington's final tally won't be official for the next few days. But the Seattle Post-Intelligencer found that support for Referendum 74 was leading 51.8 percent to 48.2 percent in the final hours of Nov. 6.
Among those to praise the preliminary results was Rick Jacobs, founder and chair of the Courage Campaign. "More and more voters are coming to know that gay people are our neighbors, our co-workers, our fellow parishioners, our family and our friends," Jacobs said in an email statement. "It won't be long at all before all loving committed couples have the freedom to marry."
Washington Gov. Chris Gregoire signed the voter-approved marriage equality legislation into law in February. Opponents of the law, which would have taken effect in June, filed more than 200,000 signatures seeking a Nov. 6 public vote on the issue.
A number of high-profile, Washington state-based corporations -- including Amazon, Nordstrom and Starbucks -- publicly backed Referendum 74, despite ample criticism from some conservative outlets and right-wing pundits.
In other big wins for marriage equality, Maine and Maryland legalized gay marriage. In Minnesota, voters shot down a constitutional amendment that would have defined marriage as a union only between a man and a woman.