The donation was announced Tuesday by Washington United for Marriage, which is working to approve Referendum 74. The donation was made on Oct. 15, but didn't publicly post online with the Public Disclosure Commission until Tuesday. Referendum 74 asks voters to either approve or reject the law allowing same-sex marriage that was passed by the Legislature earlier this year.
Bill Gates had made a $100,000 contribution to the campaign earlier this year.
About $11 million has been spent on the campaign so far, with a bulk of it spent by gay marriage supporters. Washington United for Marriage has far outraised its opponents, bringing in nearly $11 million compared to the more than $2 million raised so far by Preserve Marriage Washington, which opposes the law.
A poll released last week showed that 56.3 percent of registered voters back gay marriage in the state, compared to 35.6 percent who oppose it, with 6.1 percent undecided.
Among likely voters, support decreased to 54.1 percent, with 38.4 percent saying they would vote against the measure, and 5.7 percent undecided.
The poll, conducted by the University of Washington, also included a third prediction, based on whether people answered honestly. Matt Barreto, director of the poll, said that because sometime people answering poll questions feel social pressure to answer a certain way, results can be skewed. In this poll, they weighted the poll based on how they answered two additional questions: if they lied on the survey and if any topics made them uncomfortable. That third prediction reduced those supporting the referendum down to 52.9 percent, and those opposing 46.6 percent.
Barreto noted that in 2009, when voters were deciding on a referendum on the state's so-called "everything but marriage" expansion of the domestic partnership law, his polling showed the measure was up by 17 points, but it ultimately only won by just over 6 percentage points.