The last time voters in Bristol County, Massachusetts had more than one candidate for district attorney on their ballots was 2006. For the following sixteen years, the power of choosing which crimes to prosecute belonged to Sam Sutter and his first assistant Thomas Quinn III, who was appointed district attorney after Sutter left office in 2015.

Quinn will seek re-election for a third term this year and, for the first time, he is expected to face a challenger.

Shannon McMahon, a former Bristol County prosecutor who worked under Quinn from 2014-2016, announced her candidacy for D.A. on Wednesday.

McMahon said one of her top priorities would be expanding the county’s drug court, which aims to steer people arrested for petty crimes or drug possession into treatment rather than jail.

As a prosecutor, McMahon handled cases in New Bedford’s drug recovery court at a time when fentanyl was first emerging in the U.S. as a recreational drug. Within a matter of months, the synthetic opioid drastically raised the risk of death for people buying heroin or painkillers on the street, especially in Fall River and New Bedford, two cities in Bristol County that already led the state in the rate of opioid overdose deaths.

“Very few times as a prosecutor do you actually see what's called a success, because after you prosecute you never even see the people again, or you see them again for another crime,” McMahon said. “With the drug court, you actually saw people get better.”

If elected D.A., McMahon said she hopes to fund the creation of similar courts for veterans and people suffering from mental health crises.

Still, McMahon is quick to distance herself from the wave of progressive prosecutors that recently won election in Boston, San Francisco and Philadelphia on promises to minimize the use of incarceration.

“I'm not a progressive D.A. I would say I'm a realist,” McMahon said. “Being tough on crime, in my opinion, is not just prosecuting a case. Being tough on crime means you don't get a verdict and then walk away.”

McMahon, a 41-year-old resident of Swansea, plans to run as a Democrat in a September primary against Quinn. She currently works as a trial attorney for an insurance company.

In an interview about his re-election campaign, Quinn, who is 61 and lives in Fall River, pointed to his success prosecuting high profile murders — a type of case that McMahon, during her two years in the DA’s office, has never handled.

“I'm particularly proud of a 95% solve rate of homicides that have been committed in Bristol County over the last seven years, which is well above the national average,” Quinn said.

Since taking office in 2015, Quinn has also created a special unit of the DA’s office devoted to solving cold cases. His office recently secured a conviction against a serial rapist whose crimes went unsolved for decades.

Prosecutors in Bristol County also recently filed charges against a suspect in a 19-year-old unsolved murder case from New Bedford. A spokesman for the D.A. said investigators cracked the case by finding overlooked DNA evidence in a conch shell that was allegedly one of several weapons used to bludgeon a woman to death in her home in 2001.

As of Wednesday, McMahon and Quinn are the only candidates from either party who have signed out nomination papers for September’s primaries. The window to file enough signatures to get onto the ballot closes on May 3.

Ben Berke can be reached at Follow him on Twitter @BenBerke6.