Microsoft co-founder Bill Gates and his wife, Melinda, have donated $1 million to a Washington state campaign seeking to expand background checks on gun sales, bringing the total amount the campaign has brought in up to nearly $6 million.

The donation to the Initiative 594 campaign was given Friday, but it was not made public until Monday, when it posted on the state's Public Disclosure Commission website.

I-594 would require background checks for all gun sales and transfers in Washington state, including at gun shows and for private sales. Under the measure, exemptions would exist, including gifts within a family and antiques.

In a joint written statement issued Monday, the couple said that they believe I-594 "will be an effective and balanced approach to improving gun safety in our state by closing existing loopholes for background checks."

The large donation comes on the heels of fellow Microsoft co-founder Paul Allen's $500,000 donation earlier this month. Also last week, venture capitalist Nick Hanauer donated an additional $1 million, bring his total donation to the campaign to nearly $1.4 million.

Other prominent figures who have made large donations to the campaign include former Microsoft CEO Steve Ballmer and his wife, Connie, who have given $580,000.

A rival campaign, Initiative 591, would prevent the state from adopting background-check laws that go beyond the national standard, which requires the checks for sales by licensed dealers but not for purchases from private sellers. That campaign has raised just over $1 million so far.