Amazon.com founder and CEO Jeff Bezos announced a gift Friday of $2.5 million US to the campaign to defend Washington state's gay marriage law, weighing in on an issue that is hotly debated in many places across the U.S.

Last month, Microsoft CEO Steve Ballmer and co-founder Bill Gates each donated $100,000 to the campaign supporting the law. Washington United for Marriage says it has raised more than $5 million for Referendum 74, a state measure on the ballot in the upcoming federal elections in November asking people to approve or reject a law that would legalize same-sex marriage.

The referendum was put on the ballot after gay marriage opponents turned in more than 240,000 signatures. It seeks to overturn the gay marriage law signed in February by governor Chris Gregoire. The law was supposed to take effect June 7 but has been put on hold.

Gay marriage is being battled over in several states, and U.S. President Barack Obama put a spotlight on the issue this year when he expressed his support for same-sex marriage.

Maryland legalized gay marriage this year, but it's poised to have a public vote this fall. In Maine, voters will decide on an initiative to approve same-sex marriage. And in Minnesota, voters will decide whether to pass a constitutional amendment to ban gay marriage.

Same-sex marriage is legal in New York, Connecticut, Iowa, Massachusetts, New Hampshire, Vermont and Washington, D.C.