SEATTLE — Voters have passed a measure to expand background checks on gun sales and transfers in Washington state.

Initiative 594 was one of two rival gun measures on Tuesday’s ballot. It requires background checks on all sales and transfers, including private transactions and many loans and gifts. It had especially strong support in King County.

The rival measure, Initiative 591, would prevent the state from expanding checks beyond the national standard. It was trailing statewide.

Like federal law, Washington law requires checks for sales or transfers by licensed dealers but not for purchases from private sellers, like those who sell at gun shows or to friends

“Tonight’s victory of Washington’s 594 proves that citizens, including gun owners, do want common-sense gun safety laws,” Tacoma Mayor Marilyn Strickland said in a written statement.

Recent polling showed a significant advantage for the expansion measure — which also had an advantage in fundraising. If both managed to pass, officials said, the courts or the Legislature would likely have to sort it out.

Lisa Fain, 41, of Mercer Island, said she was interested in the gun background check issue.

“What happened in Marysville was very unfortunate. If there’s a silver lining, it’s perhaps that people realize we need stronger gun laws,” said Fain, referring to the deadly shooting last month at Marysville-Pilchuck High School.

Karen Smith, 40, of Bellevue, voted against the expansion and for the anti-expansion measure and called the decision very complicated.

“Guns are big in our family. We want to make sure everybody has the right to have a gun safely,” Smith said.

No other state has a gun-related measure on the ballot this year, and millions of dollars have been pouring into the state, mostly in support of expanding background checks.

Supporters of expansion have raised more than $10.3 million, with large donations from several prominent proponents like Bill and Melinda Gates and Paul Allen, a Microsoft co-founder. Everytown for Gun Safety, a group funded by former New York Mayor Michael Bloomberg, donated more than $1 million to the campaign, and has spent nearly $1 million more through its own political action committee. Bloomberg has separately donated an additional $285,000.

The National Rifle Association raised nearly half a million dollars to fight the expansion measure, but did not endorse I-591, which spent just over a million dollars on its campaign. The anti-expansion measure also seeks to prohibit confiscation of firearms without due process.

Six states — California, Colorado, Connecticut, Delaware, New York, Rhode Island — plus Washington, D.C., require universal background checks for all sales and transfers of firearms, according to the National Conference of State Legislatures. Other states have varying laws on expansion beyond what federal law requires, including Oregon, which requires a background check for purchases at gun shows.

Opponents of universal background checks take the most issue with language that would require checks for many gifts and loans. The measure has exceptions for emergency gun transfers concerning personal safety, gifts between family members, antiques and loans for hunting.

Both measures started out as initiatives to the Legislature. Lawmakers didn’t take action, but under state law, the measures were guaranteed a place on the statewide ballot.