Microsoft co-founder Bill Gates and his wife, Melinda, have donated $500,000 to Washington United for Marriage, the coalition of groups working to approve Referendum 74, which asks voters to uphold – or reject – a gay marriage law approved by lawmakers.

The AP reported that the donation was made on October 15, but was made known on Tuesday.

Gates also contributed $100,000 to the campaign earlier this year.

The marriage law, which was approved by lawmakers at the urging of Governor Chris Gregoire, was set to take effect in June but it was put on hold when opponents turned in more than twice the number of signatures needed to put the law up to a public referendum.

Gates' gift is the latest in a series of large donations. On Monday, New York City Mayor Michael Bloomberg gave $500,000 to pro-gay marriage groups in 3 states – topping his giving to the state campaigns at $750,000. Amazon CEO Jeff Bezos has donated $2.5 million to Washington United for Marriage.

Other high-profile large donors include Paul Singer, the founder of New York-based hedge-firm Elliot Management, and Paul Tagliabue, the former commissioner of the National Football League (NFL).