OLYMPIA — Big donors supporting background checks for gun buyers have so far come from the region’s particularly well-heeled: Bill Gates, Nick Hanauer, Steve Ballmer and Paul Allen.

Now billionaire former New York City Mayor Michael Bloomberg is joining the group. His gun-violence prevention group is kicking in $1 million to support the Washington state ballot measure that would expand background checks.

The donation from Everytown for Gun Safety to the measure’s backers, Washington Alliance for Gun Responsibility, appeared Friday afternoon in state campaign-disclosure records.

Initiative 594 to extend background checks to purchases made at gun shows and through personal transfers will go to voters in November.

Bloomberg is reportedly planning to give $50 million to Everytown for Gun Safety to advocate for background checks and other gun-violence prevention measures around the country. The group intends to target more than a dozen states, according to a recent New York Times report.

Everytown for Gun Safety President John Feinblatt said in a statement that his group “is proud to support the Yes on 594 campaign, which will keep Washington communities safer and respect Second Amendment rights by making sure that everyone in Washington passes a background check when buying a gun.”

The Washington Alliance for Gun Responsibility, which has now raised about $7 million total, issued a statement that read, in part: “We are grateful that Everytown’s coalition of Washington mayors, moms and 45,000 grassroots supporters is with us in the fight to pass this sensible measure.”

Gun-rights advocates have their own measure on the fall ballot, Initiative 591. That measure would bar the state from seizing guns and conducting background checks, which could potentially nullify I-594 or set up a legal fight.

The Bellevue-based gun-rights group, Citizens Committee for the Right to Keep and Bear Arms, sees the Everytown donation as an example of wealthy elites throwing their weight around to sway an election.

“It looks more like ‘Every Billionaire for Gun Control’ wants to buy this election,” wrote Alan Gottlieb, the committee’s chair, who earlier this week publicly challenged Gates to debate him over I-594. “Voters should be very concerned that their rights are for sale.”

Opponents of I-594 have raised considerably less money so far. A National Rifle Association (NRA) independent-expenditure group received about $25,000 from its parent organization, according to state campaign-finance records. Another group, Washington Citizens Against Regulatory Excess, has received $63,000 in funding, almost all of that from the Citizens Committee.

Independent-expenditure committees for initiatives like I-594 and I-591 have no limit on the size of donations they can receive, although they must report donations regularly to the state.

Joseph O’Sullivan: 360-236-8268 or josullivan@seattletimes.com