The leader in Los Angeles for the so-called progressive prosecutor movement is in some ways an unlikely choice. George Gascón is not a young public defender, like Tiffany Cabán, who came within a hair of winning the district attorney post in Queens, or Chesa Boudin, who upset the establishment choice in San Francisco last November. Mr. Gascón, 65, is a former beat cop who rose to become an assistant chief in Los Angeles and then chief of two other departments. For decades, he was part of a system he has since renounced as unfair, indifferent and ineffective.

During almost nine years as San Francisco’s district attorney, Mr. Gascón championed practices that have become central to the new breed of prosecutors — promoting alternatives to jail; gathering and relying on transparent data; limiting cash bail; expunging marijuana convictions; and deciding charges based on race-blind information. He sponsored a state initiative that reduced many drug crimes from felonies to misdemeanors and supported a proposal to tighten the legal standard for the use of deadly force by the police. Although he stopped short of the more sweeping reforms embraced by some prosecutors and while racial disparities persist in San Francisco, Mr. Gascón is in many ways the godfather of the nascent movement.

In 2018, he announced he would retire to Los Angeles, the county where he grew up after fleeing Cuba with his family. He soon became a logical choice for reformers seeking a high-profile candidate to challenge the incumbent.

His opponent in the election, Jackie Lacey, a career prosecutor with strong law-and-order bona fides, made history in 2012 when she became the first woman and the first African-American elected district attorney in Los Angeles. Eight years later, black civil rights activists who cheered that breakthrough became her harshest critics. Black Lives Matter and other groups have picketed her office weekly and disrupted public events. They criticize her decision to seek the death penalty (in 22 cases), failure to charge police officers who have killed civilians, a weak record on public corruption and a cautious approach to high-profile defendants. Those criticisms are echoed by Mr. Gascón’s prominent black endorsers, who include Representative Maxine Waters, Senator Kamala Harris and the musician and activist John Legend.

Multiple factors influence crime rates and convictions, many beyond the control of prosecutors. But by any measure, Los Angeles has relied on incarceration far more heavily than almost any other jurisdiction in California, sending people to prison at the rate of about 608 per 100,000 residents. San Francisco ranks at the other end of the scale, incarcerating about 130 people per 100,000.