District 3 Councillor Frank Baker, who developed a reputation over six terms for delivering constituent services and sometimes clashing with mayors and his colleagues on the City Council, isn’t running for reelection.

The move opens up a seat for a recently reconfigured district that curves down from the Ink Block complex in the South End to Gallivan Boulevard in Dorchester’s Neponset neighborhood. The district now includes more of Fields Corner, which has long been a Vietnamese residential and commercial stronghold.

The field has two declared candidates, a teacher and a community activist, while others are said to be considering a run, including an attorney who worked as Sen. Elizabeth Warren’s campaign field director in 2012, a former Beacon Hill aide, a Boston Public Schools adviser who managed Marty Walsh’s first mayoral campaign, and a leader in the Cape Verdean community.

“The privilege and responsibility of serving as a city councilor requires my round-the-clock dedication,” Baker said in a statement Thursday. “I cannot fulfill my obligation in any other way than 100% committed to the duty of this role. For an assortment of reasons, now is the right time for me to move on to the next chapter of my life, with my family.”

Dorchester state Rep. Dan Hunt praised Baker, saying he followed in the footsteps of two predecessors, Maureen Feeney and James Byrne, and “sacrificed his family time for the neighborhood and Dorchester.”

Voters first sent Baker to City Hall in 2011, in a rough-and-tumble race that became a proxy battle between Mayor Thomas Menino and then-state Rep. Marty Walsh. People aligned with Menino, who shut down the city printing department where Baker had worked, supported Cedar Grove’s John O’Toole, while Walsh backed Baker, his friend from childhood.

Steve Bickerton Jr., the head of the Cedar Grove Civic Association, said the exiting councillor performed “countless acts of help, advocacy and service” in the district. “He knows if a family goes through a tough time, if he can be helpful, and he was always there to share good news as well, like if a project was moving forward,” he said.

Baker also emerged as a critic of Mayor Michelle Wu and aligned himself with three other councillors (Michael Flaherty, Erin Murphy and Ed Flynn) as the 13-member body has split along political, racial and generational lines.

The clashes over the redrawing of political boundaries were particularly bitter. As part of the process known as redistricting, councillors voted 9 to 4 for a map that primarily shifted the boundaries of South Boston-based District 2 and Dorchester’s District 3. Several precincts moved from District 3 to District 4, resulting in the fracturing of Adams Village, and for Baker, the loss of some strongholds of conservative-leaning voters.

The changes enraged Baker, who took to the Council floor to verbally attack Liz Breadon, the redistricting chair and Allston-Brighton councillor. Wearing a Celtic cross pin on his lapel, Baker claimed to have spoken with a priest who said she was a Protestant making an “all-out assault on Catholic life.” Breadon fired back that what he said was an “insult” and an “absolute disgrace.”

Ed Cook, chair of the Ward 15 Democratic Committee, said he was once an enthusiastic Baker supporter, but expressed disappointment in Baker’s behavior and added he was mystified by the councillor’s intense focus on redistricting.

A federal judge is now weighing a challenge to the map, partially funded by Baker, who put $10,000 from his campaign account for the legal fight.

Baker’s decision leaves District 3 with two candidates vying for an open seat, though others are likely to surface in the days to come. Joel Richards, an educator backed by the Boston Teachers Union, and Jennifer Johnson, a community activist, are the two declared candidates, who jumped into the race before Baker decided against another two-year term.

“I applaud Councilor Baker on his dedicated service to our city and wish him and his family well,” Richards said Thursday. “I got into this race because the people of district three deserve a councilmember who will fight to lower housing costs, improve our schools, and help our small businesses.”

Johnson, the president and a leader of Meetinghouse Hill Civic Association for a decade, said she wished Baker well. She noted she often worked behind the scenes with him. “Now I look forward to putting my years of experience in District 3 to work for my neighbors around the urgent issues facing our City, including housing, public education, and delivering city services equitably,” she said.

Other candidates could emerge in the coming days. Sources told the Reporter that Megan Costello, currently working as a senior adviser to Boston Public Schools, is eyeing a campaign. A longtime Walsh aide, she managed his 2013 mayoral campaign.

Marty Martinez, an Uphams Corner resident who served as Mayor Walsh's chief of health and human services, said he's received several encouraging texts and emails since Baker's announcement Thursday morning. "I’m giving it a consideration, it’s fair to say," he said.

Savin Hill’s Matt Patton, an attorney and former field director for Sen. Elizabeth Warren, is also said to be considering a run. Shirley A. Jones, a member of the Ward 15 Democratic Committee, could also be a contender.

Adams Village’s Pat O’Brien, a veteran who served as a staffer to former state Sen. Linda Dorcena Forry and current state Sen. Nick Collins, said this morning he is mulling a District 3 campaign, while Bowdoin Geneva’s Paulo De Barros, director of the Cape Verdean Association of Boston, is also in the mix as a potential candidate.

Ten years after his campaign for the District 3 seat, John O’Toole’s phone has started ringing again. But the longtime civic activist told the Reporter he is likely sitting this race out.

He made peace with Baker after 2011, and “I appreciate the work that Frank did in the district,” O’Toole said.

He’ll be tracking the District 3 race closely from the sidelines. “The Council’s become a bit chaotic these days,” O’Toole said. “I’m hoping someone can bring back some civility and unity.”

News Editor Seth Daniel contributed to this report. This post is being continuously updated throughout the day.