Cal. Const. art. II, § 9

(a) The referendum is the power of the electors to approve or reject statutes or parts of statutes except urgency statutes, statutes calling elections, and statutes providing for tax levies or appropriations for usual current expenses of the State.

(b) A referendum measure may be proposed by presenting to the Secretary of State, within 90 days after the enactment date of the statute, a petition certified to have been signed by electors equal in number to 5 percent of the votes for all candidates for Governor at the last gubernatorial election, asking that the statute or part of it be submitted to the electors. In the case of a statute enacted by a bill passed by the Legislature on or before the date the Legislature adjourns for a joint recess to reconvene in the second calendar year of the biennium of the legislative session, and in the possession of the Governor after that date, the petition may not be presented on or after January 1 next following the enactment date unless a copy of the petition is submitted to the Attorney General pursuant to subdivision (d) of Section 10 of Article II before January 1.

(c) The Secretary of State shall then submit the measure at the next general election held at least 31 days after it qualifies or at a special statewide election held prior to that general election. The Governor may call a special statewide election for the measure.