SEATTLE — The nation’s only major gun-control ballot initiative this year, here in Washington State, is drawing big money and star-power names like Bill Gates and former Mayor Michael R. Bloomberg of New York as backers. But the defining aspect is confusion, many residents say, because there are in fact two gun measures on the ballot, side by side, and they are diametrically opposed.

One proposal, Initiative 594, would impose background checks of gun buyers that go beyond federal law, requiring them in private sales and at gun shows. The other, Initiative 591, would expressly forbid state expansion of background checks beyond the current federal law.

If both pass, they would effectively cancel each other out like matter and antimatter, throwing the question to the Legislature or the State Supreme Court, election officials said, neither of which would have a legal road map to guide them. Polls show that considerable numbers of people, out of confusion or strategic thinking, plan to vote yes on both or no on both. It is also possible that many people, in frustration, will walk away without voting on either measure.

The back-and-forth has kept Nikolas Peterson from filling out his ballot at all, with only a few days to go before the vote-by-mail election.