SAN FRANCISCO — Twenty years ago, amid a national panic over crime, California voters adopted the country’s most stringent three-strikes law, sentencing repeat felons to 25 years to life, even if the third offense was a minor theft.

The law epitomized the tough-on-crime policies that produced overflowing prisons and soaring costs.

Now California voters appear poised to scale back the heavy reliance on incarceration they once embraced, with a measure that would transform several lower-level, nonviolent felonies into misdemeanors punishable by brief jail stays, if that, rather than time in a state penitentiary. The referendum on Nov. 4 is part of a national reappraisal of mass incarceration.

To its advocates — not only liberals and moderates, but also an evangelical conservative businessman who has donated more than $1 million to the campaign, calling it “a moral and ethical issue” — the measure injects a dose of common sense into a justice system gone off the tracks.

“Law enforcement has been on an incarceration binge for 30 years, and it hasn’t worked,” said George Gascón, the San Francisco district attorney and a former police chief who, bucking most of his counterparts around the state, is the main sponsor along with a former police chief of San Diego. For the large numbers of nonviolent offenders with mental health or substance abuse problems, Mr. Gascón said, “Incarceration doesn’t fix the problem.”