SEATTLE — A judge struck down Tim Eyman’s latest tax-limiting measure Thursday, finding among other problems that it was a thinly disguised effort to propose a constitutional amendment — which can’t be done by initiative in Washington.

The decision from King County Superior Court Judge William Downing was an overwhelming win for Eyman’s opponents, who prevailed on their major arguments, but it’s certain to prompt an appeal to the state Supreme Court.

Voters last fall narrowly approved Initiative 1366, which would cut the sales tax by 1 percentage point, beginning in April, unless lawmakers allow a public vote on an amendment that would require a two-thirds supermajority in the Legislature for future tax increases. The sales tax cut would be a drastic hit to state revenue, costing an estimated $8 billion through the middle of 2021 at a time when lawmakers are struggling to boost spending on education and mental health.

“It is solely the province of the legislative branch of our representative government to ‘propose’ an amendment to the state constitution,” Downing wrote. “That process is derailed by the pressure-wielding mechanism in this initiative which exceeds the scope of initiative power.”

The lawsuit was brought by a group of taxpayers, two Democratic lawmakers and the League of Women Voters of Washington, who argued that constitutional amendments can’t be proposed by initiative and the measure violates the rule that initiatives be limited to a single subject. The judge agreed on both points and found that the measure would “deprive legislators, individually and collectively, of their rights and duties.”

For example, he said, lawmakers would not be allowed to consider the specific terms of a constitutional amendment, or change the two-thirds requirement to, say, a 60 percent supermajority for tax increases.

Gov. Jay Inslee said he appreciated the quick ruling and said the decision will allow lawmakers to focus on the tasks at hand. Democratic Sen. Reuven Carlyle, a plaintiff in the lawsuit, said that the ruling “respects the integrity of the Constitution and it puts that integrity above any partisan politics, any one issue or any personality.”

“Protecting the integrity of the Constitution matters. It’s not always popular, but it matters,” he said.

During a hearing on Tuesday, an attorney for the state, which is defending the voter-approved law, argued that the main thrust of the measure was the sales tax reduction. Because lawmakers were not required to take any further action, the initiative was legitimate, she argued.

But Downing found that there was no way to know whether either measure — the request for a two-thirds constitutional amendment or the sales tax reduction — would have passed standing alone. The Constitution’s prohibition on having two subjects in a single initiative is designed “to ensure that enacted legislation has won approval on its own merits and not those of some other thoroughbred to which its wagon may be hitched.”

Eyman was testifying before a Senate panel already considering a two-thirds constitutional amendment when he received a text from his lawyer with the news.

“We just got a ruling,” he told the panel. “Real time text from my attorney, who says so eloquently, ‘We lose.”’

Eyman said: “We obviously disagree with the judge and his decision. but it does not change what the voters decided and I would certainly encourage this Legislature to move forward with it as it goes upward to the Supreme Court.”

Eyman, a longtime anti-tax activist, has previously sponsored initiatives requiring a supermajority vote on taxes. The state Supreme Court struck down that requirement in 2013, saying it was unconstitutional. Eyman has filed another initiative to limit tax increases to one year unless they’re approved by voters.

Opponents sued last summer in an effort to keep I-1366 from going on the ballot in the first place. The Supreme Court declined to block it, however, saying its legality was unclear and could be sorted out after the election.

Sen. Pam Roach, R-Auburn, is the primary sponsor of the two-thirds constitutional amendment and chairwoman of the committee where the proposed amendment was heard Thursday.

“What a slap in the face to voters,” she said after the hearing. “‘You are not capable of deciding what goes in your Constitution’ — that was the message that was just given by the judge.”

Roach said she expects her joint resolution to reach the Senate floor for a vote.