With more than 36% of the ballots tallied, the measure was ahead by a 70-30 margin, according to the California Secretary of State's office.
The measure makes shoplifting a felony for repeat offenders and increases penalties for some drug charges, including those involving the synthetic opioid fentanyl. It also would give judges the authority to order people with multiple drug charges to get treatment.
Proponents said the initiative is necessary to close loopholes that have made it challenging for law enforcement to punish shoplifters and drug dealers. The measure will also help the state address the homelessness and drug crises, they said.
California Proposition 36 on November ballot would increase penalties on drug, theft crimes
Opponents, including Democratic state leaders and social justice groups, said it would disproportionately imprison poor people and those with substance use issues rather than target ringleaders who hire large groups of people to steal goods for resale online. The initiative would also take away drug and mental health funding that comes from savings from incarcerating fewer people.
The measure would essentially unwind Proposition 47, which voters approved a decade ago and made some theft and drug crimes misdemeanors instead of felonies.
Earlier this year, Gov. Gavin Newsom spoke out against Prop. 36, saying Prop. 47 has saved taxpayers more than $816 million through reduced incarceration costs for minor crimes.
"Prop. 36 takes us back to the 1980s, mass incarceration, it promotes a promise that can't be delivered," Newsom said. "I would ask those who support it, particularly mayors: Where are the treatment slots, where are the beds? Twenty-two counties don't have one residential treatment facility. Twenty-two counties don't have one. I mean, they're lying to you."
Prop. 36 would also create a new category of crime called "treatment-mandated felony" where the person charged could go into treatment instead of prison.
Supporters said rampant crime throughout the state increases the need for harsher penalties.
Rick Caruso, the billionaire developer and former Los Angeles mayoral candidate, ran ads in support of the measure.
"There's two things that people are feeling," Caruso said earlier this year. "One is, they see that there's more crime around them and two is that they feel unsafe. So, what you have on the ballot with Prop. 36 is an opportunity to change that."
"A cornerstone of good government is to try something and when it's not working, pivot and try and rectify it. (Prop.) 36 does that by holding serial criminals accountable," he added.
The Associated Press contributed to this report.