Here’s the opinion in League of Women Voters v. Utah State Legislature (h/t Ballot Access News). A key excerpt:

The novel question before us asks: what happens when Utahns use their initiative power to exercise their “right to alter or reform their government” by passing an initiative that contains government reforms, and the Legislature repeals it and replaces it with another law that eliminates the reforms the people voted for? . . . .

[W]e hold that when Utahns exercise their right to reform the government through a citizen initiative, their exercise of these rights is protected from government infringement. This means that government-reform initiatives are constitutionally protected from unfettered legislative amendment, repeal, or replacement. Although the Legislature has authority to amend or repeal statutes, it is well settled that legislative action cannot unduly infringe or restrain the exercise of constitutional rights. Consequently, when Utahns exercise their right to reform the government through an initiative, this limits the Legislature’s authority to amend or repeal the initiative. . . .

In this case, Plaintiffs claim. . . that Utahns used their initiative power as a means of exercising their right to reform the government when they passed Proposition 4. And they claim that the Legislature violated those rights when it enacted S.B. 200, which repealed Proposition 4 and replaced it with a new law that nullified Proposition 4’s key provisions. The Legislature’s general legislative power to amend, repeal, and enact statutes does not defeat this claim as a matter of law.

Update: The AP has this story, with more background on yesterday’s decision.