Cal. Gov. Code § 9512

When a bill has passed both houses of the Legislature and is returned by the Governor without his signature, and with objections thereto, or if it be a bill containing an item or several items of appropriation which is returned with objections to one or more items, and upon reconsideration the bill, item, or items pass both houses by the constitutional majority, the bill, item, or items shall be authenticated as having become a law by a certificate.