SACRAMENTO — Conceding that a tough-on-crime law he signed four decades ago had failed miserably because of “unintended consequences,” Gov. Jerry Brown on Wednesday unveiled a ballot measure aimed at allowing nonviolent felons to seek early release and shrinking the number of juveniles tried as adults.

If voters approve the initiative in November, thousands of state prison inmates who have already completed their basic sentence and passed a public safety screening would become eligible for parole. And for the first time, offenders who complete rehabilitation programs while behind bars could earn credits for their efforts.

The measure would also allow judges rather than prosecutors to decide whether juveniles as young as 14 should be tried as adults. The judges lost that power under Proposition 21, an anti-crime measure approved by California voters in 2000.

The governor pledged to do “whatever it takes to get this done.”

Because Brown is widely expected to use $24 million in leftover campaign cash to finance the gathering of 585,407 valid signatures, the measure is almost certain to get on an already crowded ballot in which Californians will likely be asked to decide whether to legalize marijuana, kill the death penalty and dramatically boost the minimum wage.

Brown made the announcement on an afternoon conference call with reporters that featured several law enforcement officials and a representative of the Catholic Church, all of whom spoke in favor of the proposal.

Criminal justice reform advocates uniformly praised the measure and said they’re cautiously optimistic it will pass, noting that similar reform measures passed in each of the last two election cycles.

“It’s really, really exciting,” said Lenore Anderson, head of Californians for Safety and Justice, which sponsored 2014’s Proposition 47, an initiative that reduced penalties for some nonviolent crimes. “It’s a remarkable development that will do the state a lot of good.”

But while Brown will have support from some law enforcement officials, including an unlikely alliance with San Diego County’s Republican district attorney, he won’t have the support of many of them.

“My guess is we’ll oppose it,” said Sean Hoffman, director of legislation for the California District Attorney’s Association. “But we’re still unpacking what it does and probably won’t decide officially until next week.”

The Assembly Public Safety Committee’s top Republican said her caucus will oppose the initiative because recent changes in sentencing laws have already caused spikes in crime in parts of California. And if the governor’s proposal becomes law, she said, it would only exacerbate that problem.

“Assembly Republicans support giving local officials the tools they need to properly rehabilitate criminals,” said Melissa Melendez, R-Lake Elsinore. “However, protecting Californians and our communities must come first.”

But Santa Clara County Public Defender Molly O’Neal said she’s “wildly in favor of” the proposed changes, especially the piece of the measure that would reduce the number of juveniles sentenced in adult court.

“With advancements in science around adolescent brain development, we know that youth don’t fully develop until age 25,” she said.

According to the international group Human Rights Watch, California is one of 15 U.S. states that allow a prosecutor instead of a judge to make the decision to file cases in adult court. Since 2003, Human Rights Watch said, more than 10,000 California offenders under 18 have been tried as adults — and 7,200 of them are directly filed in adult court with no oversight by a judge.

Stanford law Professor Joan Petersilia, a criminal justice expert, said close to 20,000 inmates convicted of drug and property crimes would eventually qualify for early release if the measure becomes law.

She said the shift in policy could be “hugely transformative” for those prisoners because many are serving sentences that were extended by “enhancements” for things like gang affiliation.

“Systematically releasing the least risky prisoners is very smart,” she said.

In the late 1970s, Brown signed a law that replaced California’s famously flexible sentencing policies with the strict guidelines judges use today. So-called “determinant sentencing” eliminated any incentive for inmates to improve themselves and led to increased rates of recidivism, Brown said Wednesday.

By 2003, when he was Oakland mayor, prisons had become so crowded that Brown told the Little Hoover Commission the reform he signed had turned into an “abysmal failure,” giving inmates facing long, fixed terms little incentive to reform themselves.

“The prisons started building up about the time I was leaving,” Brown said in an interview in 2010 when he was still attorney general. “But they didn’t stop. They just kept on going. We see now that the determinate sentence, which I signed, needs substantial revision.”

The state’s prison system became a major issue for Brown when he began his third term as governor five years ago with a court order to reduce crowding. Brown that year enacted prison realignment, in which the state shifted responsibility for certain low-level offenders from the prison system to counties.

Last year, Brown vetoed a slate of crime-related bills, including one focused on date rape, complaining in a veto message about a “multiplication and particularization of criminal behavior” that he said would make the state’s penal code even more complex.

“Over the last several decades, California’s criminal code has grown to more than 5,000 separate provisions, covering almost every conceivable form of human misbehavior,” Brown wrote then. “During the same period, our jail and prison populations have exploded.

“Before we keep going down this road, I think we should pause and reflect on how our system of criminal justice could be made more human, more just and more cost-effective.”