* "We think it's the largest ever" contribution
* Issue on the ballot this November
By Elaine Porterfield
SEATTLE, July 27 (Reuters) - Amazon Inc founder Jeff Bezos and his wife have made what may be the biggest donation to the cause of gay marriage, contributing $2.5 million to a Washington state campaign to legalize same-sex nuptials there, the campaign said on Friday.
The donation to Washington United for Marriage comes as the question of deep-pocketed executives taking a stance on gay marriage has risen in prominence, with the mounting controversy over the president of Chick-fil-A's public opposition to same-sex marriage.
Bezos, who is chief executive of Amazon, and his wife, MacKenzie, h ad until now kept a lower profile on charitable and political contributions than other prominent high-tech billionaires in Washington state, such as Microsoft co-founder Bill Gates and his wife, Melinda.
Andy Grow, a spokesman for Washington United for Marriage, said his group had checked with other campaigns and gay rights advocates were not aware of a larger donation.
"We think that it's the largest ever," he said of the Bezos' donation.
Washington United for Marriage is campaigning for Referendum 74, which if approved would affirm a law that passed the Democratic-led state Legislature in February and would have legalized gay marriage. The law was blocked from taking effect when opponents gathered enough signatures to put it on the November ballot.
"This is a history-making gift," Washington United for Marriage campaign manager Zach Silk said of the Bezos donation.
"I think it shows our goal of standing up for all Washington families is shared by many people, including Jeff Bezos and his wife," Silk said.
Gates, along with current Microsoft Chief Executive Steve Ballmer, earlier donated $100,000 to the Washington United for Marriage referendum campaign.
A representative for Preserve Marriage Washington, the group opposed to same-sex marriage in the state, could not be reached for comment.
That group has raised $253,000, according to figures on the website of the Washington state Public Disclosure Commission. Before the donation from the Bezos, Washington United for Marriage had pulled in nearly $2.5 million, the commission website said.
A poll last month by Public Policy Polling found 51 percent of Washington state voters supported gay marriage, while 42 percent believed it should be illegal and 7 percent were undecided.
Currently, six states and the District of Columbia allow same-sex marriage.
Silk said his campaign's volunteer finance chair, Jennifer Cast, sent the Bezos an email last weekend seeking their support. Cast, who is a lesbian and was a senior manager at Amazon for more than seven years, knows the couple personally.
In the email, Silk said Cast wrote to the couple: "We need help from straight people. To be very frank, we need help from wealthy straight people who care about us and who want to help us win."
On the other side of the issue, gay rights advocates and some local politicians have in recent weeks taken aim at fast-food chain Chick-fil-A and its president, Dan Cathy, after he told the Baptist Press his company supported the "biblical definition of the family unit." Same-sex couples across the country plan a kiss-in at Chick-fil-A restaurants on Aug. 3.