Unanimous decision in Legislature of the State of California v. Weber. From the introduction:

Petitioners — the Legislature of the State of California, Governor Gavin Newsom, and elector and former Senate President Pro Tempore John Burton — filed this original proceeding seeking a writ of mandate or prohibition to bar the Secretary of State (Secretary) from placing an initiative measure on the November 2024 general election ballot. The measure at issue has been designated Attorney General Initiative No. 21-0042A1 and Secretary of State Initiative No. 1935, and has been named the “Taxpayer Protection and Government Accountability Act” by its drafters. We refer to it as the “TPA.” The petition primarily contends that the TPA is invalid because it attempts to revise the California Constitution via citizen initiative. Petitioners also argue that the TPA is invalid because it would seriously impair essential government functions. Petitioners named Thomas W. Hiltachk, the proponent of the challenged measure (Proponent), as real party in interest.

…

“We stress initially the limited nature of our inquiry. We do not consider or weigh the economic or social wisdom of general propriety of the initiative.” (Amador Valley Joint Union High Sch. Dist. v. State Bd. of Equalization (1978) 22 Cal.3d 208, 219 (Amador Valley).) The only question before us is whether the measure may be validly enacted by initiative. After considering the pleadings and briefs filed by the parties and amici curiae as well as the parties’ oral arguments, we conclude that Petitioners have clearly established that the challenged measure would revise the Constitution without complying with the appropriate procedure. The changes proposed by the TPA are within the electorate’s prerogative to enact, but because those changes would substantially alter our basic plan of government, the proposal cannot be enacted by initiative. It is instead governed by the procedures for revising our Constitution. We therefore issue a peremptory writ of mandate directing the Secretary to refrain from taking any steps to place the TPA on the November 5, 2024 election ballot or to include the measure in the voter information guide.