Almost four decades after he signed a law mandating strict sentences for the most serious crimes, Gov. Jerry Brown on Wednesday moved to ease its effect, proposing inmates convicted of nonviolent offenses be given a chance at early release.

“Let’s take the basic structure of our criminal law and say, when you’ve served fully the primary sentence, you can be considered for parole,” Brown said in announcing a November ballot initiative to streamline the rules — one he estimated could affect thousands of current inmates.

Rather than change sentencing policy, the proposal would allow corrections officials to more easily award credits toward early release based on an inmate’s good behavior, efforts to rehabilitate or participation in prison education programs.

“It’s well-balanced,” Brown said. “It’s thoughtful.”

The effort is largely in response to the lingering effects of a 2009 federal order for California to reduce its prison population, Brown said. But he made clear that it also is meant to improve a criminal justice system that offers too few chances at rehabilitation.

“By allowing parole consideration if they do good things,” the governor said of some inmates, “they will then have an incentive … to show those who will be judging whether or not they’re ready to go back into society.”

Brown had been hinting for months that he was considering a key change in criminal justice policy, and consulted with a number of academics and inmate advocates on how to proceed.

He was joined Wednesday by a handful of prominent law enforcement and religious leaders.

While it was unclear whether they were ready to fully embrace each detail of the measure, they praised Brown’s focus on weeding out those serving time for nonviolent offenses.

“I think this will effectively open bed space for those who richly deserve to be there,” Los Angeles Police Chief Charlie Beck said.

The initiative also would authorize the state parole board to consider early release for nonviolent inmates who complete a full sentence for their primary offense and it would require a judge to decide whether felons as young as 14 should be tried in juvenile or adult court.

That final element of the initiative would undo a system approved by voters in 2000 that handed that power to prosecutors.

Once the measure is given a formal title and summary by the attorney general’s office, Brown and his political team will need to gather more than 585,000 valid voter signatures to qualify it for the Nov. 8 statewide ballot.

The governor likely has the needed resources: Campaign funds left over from his 2014 reelection bid and previous successful ballot measures total some $24 million.

Brown’s passion for broader reform was apparent last fall, when his veto of a package of crime bills included a call for a broader, more systemic discussion.

The new proposal, said Stanford Law School professor Joan Petersilia, would do just that by providing a necessary recalibration.

“It all is about shifting power back to those who are closest to the inmate,” Petersilia said.

Brown’s signature on the 1977 determinate sentencing law marked the beginning of a tough-on-crime era — in the Legislature and at the ballot box.

But critics argued that it also fueled a booming prison population.

On Wednesday, the governor lamented the mandatory-sentencing law’s “unintended consequences.”

“So many times politicians are not willing to go back and question major things they did in office,” Petersilia said. “He’s to be given credit…. He’s willing to admit we learned something in California and can now do a course correction.”

Some law enforcement groups and legal activists remained skeptical upon hearing early details of the governor’s proposal.

Kent Scheidegger, director of a pro-death-penalty foundation, said the initiative’s focus on only the convicting crime could lead to unwelcome outcomes.

“It says that if the person is convicted of a burglary of a home, and has prior convictions for rape and murder, he is eligible for parole as soon as he finishes his sentence,” Scheidegger said. “And that is insane.”

Patrick McGrath, district attorney of Yuba County, said Brown’s plan — by offering more pathways to parole — also may send the wrong message to crime victims who believe their perpetrators received a certain punishment.

“Now, down the line, they’re told 10 year [sentences] are not really 10 years,” he said. “I think this is very, very corrosive to the faith that the public has ultimately in the criminal justice system.”

Loyola Law professor Laurie Levenson, a former federal prosecutor, said the proposal would make a judge’s sentence only a starting point.

“People could be released from prison years earlier based on what the parole board wants to do,” she said.

Brown has long voiced regrets about signing the law. In 2003, as mayor of Oakland, he called the law an “abysmal failure” and told a Times reporter that he’d had doubts about it from the very beginning.

Of particular interest will be how Brown shapes the narrative of the political campaign in support of his parole initiative. The fall statewide ballot already is expected to be one of the longest in more than a decade, which will mean voters are deluged with a flood of advertisements, mailers and messages.

The ballot also likely will feature other high-profile public safety debates, including a gun violence initiative promoted by Lt. Gov. Gavin Newsom and perhaps dueling initiatives to either eliminate or strengthen California’s death penalty.

For his part, the governor cast the proposal Wednesday as only an opportunity for some who are now behind bars, not a guarantee.

“Its essence is to provide an incentive, both reward and punishment,” Brown said in describing how an inmate would have to earn his or her early release. “It will require an act of will.”