In a wide-open primary for Suffolk County district attorney, Democrats on Tuesday voted for a change candidate, nominating Rachael Rollins for the office that oversees prosecutions in Boston, Chelsea, Revere and Winthrop.

Rollins emerged from a five-person field of mostly progressive candidates to replace Suffolk County District Attorney Dan Conley, a Democrat who decided against running for re-election after more than 16 years in the office.

One of the top challengers called to concede around 9 p.m., according to both campaigns.

A former top transportation lawyer, Rollins supports the repeal of mandatory minimum sentences for all drug offenses and opposes the cash bail system, she wrote in a questionnaire for the American Civil Liberties Union of Massachusetts.

Liberal reformers saw a big opportunity in the open DA seat responsible for prosecuting criminals in the state's largest city, and Rollins's supporters brimmed with excitement at her Jamaica Plain headquarters as the numbers rolled in.

District attorneys have traditionally trumpeted their law enforcement bona fides and taken a more hard-line approach toward justice than their peers in other elected offices. Conley and several other DAs clashed with Bay State lawmakers over the criminal justice bill that was signed into law earlier this year, telling Senate leaders last fall that their bill, which included the repeal of several mandatory minimum sentences for drug dealers, "largely ignores the interests of victims of crime."

U.S. Sen. Elizabeth Warren last month called the criminal justice system "racist" from "front to back," and activists around the country have voiced outrage in recent years after police-involved killings of black men and women. Some of that anger has been channeled into electoral efforts to put reformers in charge of prosecutors' offices.

Last year, Philadelphia voters elected civil rights lawyer Larry Krasner as district attorney, and he has ushered in a series of reforms, such as instructing prosecutors not to charge marijuana crimes.

Rollins won the endorsement of Real Justice PAC, which is run by veterans of Vermont Sen. Bernie Sanders's presidential campaign, in her bid for district attorney. The group credited Rollins with speaking out "against close relationships between police officers and and the DA’s office" and highlighting a "need for outside investigations into officer-involved shootings."

The field of candidates included other progressives. Rep. Evandro Carvalho and former defense attorney Shannon McAuliffe also favored repealing mandatory minimum sentences, and cash bail, which can leave poor people with few options besides jail.

Rollins was the top lawyer for the MBTA, the state Department of Transportation and the Massachusetts Port Authority.

"We're going to end wealth and race disparity in the criminal justice system," Rollins told a crowd in Dorchester in August. She said, "Our prisons are debtors prisons. People who are poor, sit in jail."

Candidate Greg Henning, who headed up the gang unit for Conley, and former prosecutor Linda Champion both opposed those changes. Henning called to concede around 9 p.m., according to Rollins' campaign and Henning's campaign.

"This race means so much, because this is one of the most important and powerful positions within the criminal-legal system," said Rahsaan Hall, who directs the Racial Justice Program for the ACLU of Massachusetts, a couple weeks before the votes were counted.

While the ACLU did not endorse candidates, the civil liberties organization has tried to raise public awareness about DA races happening throughout the state.

"The last time we saw this many contested races was in 1982, which was at the height of the 'War on Drugs,'" said Hall, adding that a first-ever debate held in a house of correction between the candidates has helped raise the profile of the Suffolk DA race across the country.

In November's general election, Rollins will face independent Michael Maloney, who has worked as a defense attorney and a marijuana entrepreneur. On several issues, Maloney has staked out liberal positions on criminal justice matters.

In the questionnaire for the ACLU, Maloney said he would "cease prosecuting non-violent drug crimes, which are consistent with addiction," and he said he believes the DA contributes to racial disparities in criminal justice.

Andy Metzger is a freelance journalist based in the Boston area. He covers politics and the environment.