Before he was a congressman, Joe Kennedy was a prosecutor

Joe Kennedy III, at work as a prosecutor in court in Falmouth in 2011.Suzanne Kreiter

He was the new guy in the office, the entry-level prosecutor trudging with an armful of paperwork from his office in Barnstable to one district court after another, sometimes on a 7 a.m. flight to Nantucket to argue the motions and bail reviews for that day’s docket.

Joseph P. Kennedy III was straight out of Harvard Law School when he joined the Cape and Islands district attorney’s office full time in 2009, after a brief internship. Now 39 and running for Senate, Kennedy’s time there — and a few months in Middlesex County as a district court prosecutor — made up the bulk of his professional resume before he entered electoral politics in 2012, when he was elected to the US House of Representatives.

As he looks to unseat incumbent Senator Edward Markey in the Sept. 1 Democratic primary, Kennedy’s time as a prosecutor has become a point of contention in a race that increasingly appears to hinge on which candidate has stronger progressive bona fides. In an election season shaped in part by the death of George Floyd, a Black man killed by a white police officer in Minneapolis, the two campaigns are drawing very different conclusions about Kennedy’s years working in a criminal justice system that both candidates now see is in dire need of reform.

To Kennedy, the work was a learning experience in the realities of the Massachusetts courts, and provided an understanding of what needs fixing. His critics, led by Markey, question the progressive values of a young lawyer who chose to become a criminal prosecutor in the administration of perhaps the most right-wing district attorney in the state, Michael O’Keefe, a Republican and future supporter of President Trump.

“Congressman Kennedy had to decide where he was going to work as a lawyer, and I don’t think it was good judgment,” Markey said bluntly during a recent debate, pointing to recent protests by social justice advocates against “the status quo in the criminal justice system in our country.”

O’Keefe, in an interview, said Markey “knows nothing about me” or the diversion programs his office has spearheaded through the years.

“I don’t consider this about me. I don’t know Senator Markey, I haven’t seen him at anything in Massachusetts,” O’Keefe fired back.

He called Kennedy “a great employee” who performed the work required of an assistant district attorney, which he called a training ground for a young lawyer.

Kennedy would not discuss specifics of his caseload as a prosecutor, but said through a spokesperson that the work, along with his volunteer work during law school with the Harvard Legal Aid Bureau and his two years spent with the Peace Corps, “helped fortify his commitment to public service.”

“They also informed the issues that have come to define his congressional career, from mental health care to justice reform to economic equity,” the spokesperson, Emily Kaufman, said.

Kennedy has also earned the support of grass-roots workers across the state, including those who have pushed for police and court reforms.

“I’ve dealt with many persons who have been on the other side of the criminal justice system, whether they be prosecutors, police officers, or probation officers,” said Bishop Talbert Swan, president of the Greater Springfield NAACP. “Them being for reform, for justice, is not antithetical to their career choice. They just have to mete out justice fairly.”

But some of the state’s most progressive advocates in the criminal justice system, including Suffolk District Attorney Rachael Rollins, have endorsed Markey, touting his work toward gun reform.

What is clear, according to lawyers who practiced alongside Kennedy and know the duties of an entry-level prosecutor, is that the laborious post has little wiggle room to build a political platform, and is rather a crash course in the mechanics of the courtroom. There is a build-the-plane-as-you-fly-it mentality that requires rookie prosecutors to make daily decisions that can have enduring impacts on people’s lives, and yet they serve only as cogs in the wheels of the broader, fast-moving criminal justice system.

The life of a district court prosecutor in Massachusetts, a $40,000-something-a-year entry-level position, is meant to groom young lawyers for big cases in Superior Court or at the federal level, not necessarily a seat in Congress.

“It’s not an indicator at all of values, it’s an indicator that you’re straight out of law school learning to try a case,” said Andrea Cabral, a former state prosecutor and sheriff, and the former head of the Executive Office of Public Safety and Security. She said work as a prosecutor and a progressive advocate are not mutually exclusive — she specialized in domestic violence, civil rights, and sexual assault cases and worked to reform the way police and the courts handle them — but a prosecutor’s early years on the job are where values are developed, not displayed.

“I think everybody finds their way,” she said, “but it’s about building enough confidence in yourself, so that you can scrutinize cases.” She has not endorsed anyone in the race.

Kennedy could have been in Nantucket one day, for the arraignment of a bank robber, and perhaps Falmouth the next month for the case of a police officer involved in a drunken driving crash. Assistant district attorneys at the district court level help process an enduring flow of arraignments, bail requests, and motions for dismissals of relatively low-level crimes: disorderly conduct, DUI, drug possession. It’s the grunt work of a prosecutor’s office.

A teenage Kennedy had been on the other side of that sort of courtroom himself: When he was 17, he was arrested on charges oftrespassing after dark in a park owned by the Metropolitan District Commission, across from the Cambridge private school he attended. The charge was ultimately dismissed.

Years later, when Kennedy had discretion about how to resolve low-level crimes, he was fair, said Bill Robinson, a retired defense lawyer who headed the public defender’s office in Barnstable for two decades.

“You sometimes get young DAs. They want to build their confidence by winning cases,” he said. That wasn’t Kennedy, he said; if anything, he would see a case for what it was and use his authority to propose a reasonable outcome.

Gerard T. Leone, a former federal prosecutor and the Middlesex district attorney who hired Kennedy in summer 2011, said that he had been on track to work in Superior Court, where criminal cases are far more serious, and the outcomes far more consequential. But he hadn’t arrived at that level yet: It typically takes more than four years for a young prosecutor to graduate to Superior Court, and Kennedy was still honing what it meant to be an advocate.

“Joe enjoyed the part of giving a voice to people who don’t have a voice,” said Leone, who supports Kennedy in the race. “Whether they’re prosecutors, or defense lawyers, or civil rights counselors, there’s a base-level sense of empathy you have to have, if you’re doing it right.”

And yet the work can have implications that go beyond the day’s docket. One man that Kennedy prosecuted for drug distribution had the case overturned, because of an error at the trial. His record would have been cleared. But it was too late for the man to seek recourse; he had already been deported to his native Jamaica by the time his case was dismissed.

Tom Shack, the former head of the appellate unit in O’Keefe’s office, and Kennedy’s supervisor in Nantucket, said he does not recall the case directly but said it would not be out of the ordinary to see a case dismissed because of legal technicalities. But, he said, that does not insinuate that the prosecution was not fair, or that a crime did not occur.

“The system is not perfect, but I think that Joe is the type of person that the justice system needs,” said Shack, who considered himself a mentor of Kennedy and once brought him in as co-counsel on a high-profile murder appeal that went before the Supreme Judicial Court — a learning experience for a young prosecutor.

Jeremiah Manion of the Globe staff contributed to this report.