SEATTLE — Microsoft Corp. Chief Executive Steve Ballmer and Co-founder Bill Gates have each donated $100,000 to the campaign supporting Washington state’s recently passed law legalizing same-sex marriage, reported the Associated Press on Monday.

The law, approved by the state legislature and signed by Governor Chris Gregoire earlier this year, faces a ballot referendum in November.

The donation from Gates and Ballmer will “make a tremendous difference,” said Zach Silk, campaign manager for Washington United for Marriage, a marriage equality advocacy group working to gain voter approval for the new law.

“It’s very important for us to have that broad support from business leaders and companies themselves.”

Referendum 74 will ask voters to accept or reject the law that legalizes same-sex marriage — last month, the group “Preserve Marriage Washington,” which seeks to overturn the law, submitted petitions containing 247,331 signatures, more than twice the minimum required to qualify it for the ballot.

Prior to Gates’ and Ballmer’s donations, Washington United for Marriage had raised nearly $1.5 million for the campaign to fight back attempts to overturn the law.

Preserve Marriage Washington has raised only about $130,000 according to recent figured released by the Public Disclosure Commission, though the money race is expected to heat up significantly in the coming months.

The anti-gay National Organization for Marriage has vowed to aggressively fight to overturn the law.

Microsoft, based in Redmond, Wash., is one of many major U.S. corporations to publicly support Washington’s marriage equality bill — others include Seattle-based Starbucks, Google, Alcoa, Nike and Group Health Plan.

In 2011, Microsoft was one of 70 major U.S. companies to participate in “amicus” brief filing in the U.S. Circuit Court of Appeals for the First Circuit claiming that the federal Defense of Marriage Act is harmful to commerce.