He may be right about the causes; after all, gun violence has soared across the country in places that have traditional, tough-on-crime prosecutors as well as in those with progressive district attorneys. But Krasner must face voters later this month, and Philadelphia’s gun-violence crisis is now very much his problem. In 2017, Democrats here gave the national movement for criminal-justice reform one of its biggest victories by electing Krasner, a longtime civil-rights attorney who campaigned on policies that would reduce mass incarceration. He quickly moved to deliver on his promises, firing more than 30 veteran prosecutors, scrapping bail for a host of minor crimes, all but ending the prosecution of juveniles as adults, and declining to bring charges for most cases of simple drug possession and prostitution. He also instructed his prosecutors to seek lighter sentences and explain how much each prison term would cost city taxpayers. By the start of the pandemic last year, Krasner told me, the city’s jail population had dropped by more than 30 percent, to its lowest level since 1985, and his office had cut even further the years of parole and probation that ex-offenders spent under city supervision.