The state Supreme Court has ruled that Initiative 1366, which was approved by voters last year, is unconstitutional.

I-1366 would have reduced the state sales tax rate to 5.5 percent unless the Legislature referred a constitutional amendment to voters requiring two-thirds majority approval of tax increases.

Earlier this year, a King County Superior Court judge found I-1366 to be unconstitutional. The judge said the initiative violates Article XXIII of the constitution, which requires constitutional amendments to be proposed by the Legislature, and the single-subject rule.

The Supreme Court's majority opinion holds "that I-1366 violates the single-subject rule of article II, section 19, and that it is void in its entirety." 

The opinion notes,

Under any iteration, a reduction to the sales tax rate is unrelated to both a constitutional amendment, which would impact future legislatures, and to the way that future taxes and fees are approved.

(Emphasis in original.) Further, the opinion notes that the purpose of Article II, Section 19 is to prevent logrolling ("pushing legislation through by attaching it to other necessary or desirable legislation"). It finds that I-1366 is an example of logrolling:

In one sense, an initiative so structured warrants the protections of article II, section 19 even more than bills flawed by traditional logrolling because many voters will not even receive the benefit of at least having the provision they did vote for go into effect. . . .

The State argues that "regardless of whether some voters desired the contingency to occur, all voters affirming [I-1366] voted for the measure's sales tax reduction." Therefore, it contends, I-1366 cannot constitute logrolling because the people voted for only one provision. But this argument implicitly supports our finding of logrolling; it acknowledges that some people may have voted for the sales tax reduction because they desired the constitutional amendment. That is the definition of "logrolling." 

(Emphasis in original, citation omitted.) In addition to the single-subject violation, the opinion notes the initiative's Article XXIII problem:

Finally, the second subject of I-1366–whether it be a constitutional amendment or a change to the way all future taxes and fees are approved–alters the process for amending our state constitution, which runs afoul of article XXIII. . . .

The State argues that section 3 of I-1366 does not violate article XXIII because the legislature would still have to go through the processes outlined in article XXIII. Whether true or not, this argument fails to appreciate the "do this or else" structure of the initiative. If the legislature does not propose the amendment, it will be faced with a $1.4 billion-per-year loss in revenue. This structure, taken to its logical conclusion, establishes a new process for amending the constitution. The new norm would be for initiative sponsors to pair one drastic or undesirable measure with an ultimatum that it go into effect unless a specific constitutional amendment is proposed to the people. This new process amounts to a small percentage of voters effectuating a constitutional amendment by two majority votes and is simply not one contemplated by the constitution, even if further action is required by the legislature. Because the second subject establishes a new process for amending the constitution, it also violates article XXIII. . . .

A concurring opinion (beginning on page 24 of the ruling) argues that "this case is best resolved solely under article XXIII." 

A supermajority requirement for tax increases was first approved by voters in 1993. Since then, the requirement was suspended and reinstated several times, but the state Supreme Court ruled that it was unconstitutional in 2013. Hence the attempt by I-1366 to induce the Legislature to begin the constitutional amendment process.

Now that the Supreme Court has thrown out this attempt, it seems that the only way forward for proponents of the supermajority requirement is to elect a Legislature that will put forward such an amendment. In the meantime, budget writers must be relieved by the decision today — the sales tax reduction in the initiative would have reduced 2017-19 revenues by an estimated $3.066 billion.