With the gift, Washington United for Marriage has raised more than $5 million for its referendum campaign.
"It's a game changer for us," said campaign manager Zach Silk in Seattle. "It puts us in unique position to win."
But, his group is still the underdog, he said. In 32 previous elections nationally, same-sex advocates have lost. Silk said he believes the Washington election may be the turning point, thanks in part to the Bezos' donation.
"We're at a tipping point, and they really understand this is an historic moment, and they want to be on the right side of history and want to make history," he said.
Amazon publicly supported the law earlier this year, along with other prominent Pacific Northwest businesses, including Microsoft, Starbucks Corp. and Nike Inc.
Last month, Microsoft Corp. CEO Steve Ballmer and co-founder Bill Gates each donated $100,000 to support the law.
Amazon spokesman Drew Herdener said the Bezos' donation was a personal decision and that "Jeff and MacKenzie feel strongly about the issue."
Silk and officials with two national gay rights groups -- Freedom to Marry and the Human Rights Campaign -- say Bezos' $2.5 million donation is the largest publicly reported gift to a marriage campaign. They noted that gay marriage opponents have tried to limit the disclosure of campaign contributions, so they couldn't be certain whether there was a larger one.
Referendum 74 was certified for the November ballot in May after gay marriage opponents in the group Preserve Marriage Washington turned in more than 240,000 signatures. The referendum seeks to overturn the gay-marriage law signed in February by Gov. Chris Gregoire.
The same-sex marriage law was supposed to take effect June 7 but has been put on hold pending the November vote.
A "yes" vote on the referendum upholds the law, and a "no" vote overturns it.
Joseph Backholm, chairman of Preserve Marriage Washington, said that while he was surprised by the size of the donation, his campaign has always expected that it would be outspent by gay marriage supporters.
"We're optimistic about what's going to happen in the end," he said. "Jeff Bezos gets to vote just one time like everyone else in state. That's the great equalizer."
As of Friday, Preserve Marriage has raised just over $253,000, though Backholm said the group hopes to ultimately raise $4 million.
National groups like the Washington, D.C.-based National Organization for Marriage, which was involved in ballot measures that overturned same-sex marriage in California and Maine, have promised to help overturn the law.
Same-sex marriage is legal in New York, Connecticut, Iowa, Massachusetts, New Hampshire, Vermont and Washington, D.C. Maryland legalized gay marriage this year as well, but that state also is poised to have a public vote this fall.
In Maine, voters will decide on an initiative to approve same-sex marriage three years after a referendum overturned a law passed by the Maine Legislature. And in Minnesota, voters will decide whether to pass a constitutional amendment to ban gay marriage.
The first television ad supporting gay marriage in Washington state was set to start during the Olympics opening ceremony Friday. The statewide ad was paid for by Seattle-based Pride Foundation, and will run several times a day on broadcast television in three media markets over the entire two-week Olympic games.
Kris Hermanns, executive director of the Pride Foundation, would not disclose the cost of the ad buy, but said it "will be in the six figures."
Associated Press Doug Esser in Seattle contributed to this report.
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