Congressman Stephen Lynch took the stand in federal court this week to decry the way the City Council redrew the political boundaries of Boston’s nine district seats.

Councillors voted in November to shift South Boston public housing developments, then located in District 2, to Dorchester-based District 3, as well as parts of Dorchester, primarily Cedar Grove and Adams Village, from District 3 to District 4, which includes part of Dorchester and Mattapan.

The four councillors who opposed the redrawn district map — District 2 Councillor Ed Flynn and At-Large Councillor Michael Flaherty, both of South Boston, and District 3 Councillor Frank Baker and At-Large Councillor Erin Murphy, both of Dorchester — are now hoping to strike down the map, which was signed into law by Mayor Michelle Wu and remains in effect as city elections get underway. Most of the city’s residents remained in the same Council district.

Lynch and some local civic associations in Dorchester and South Boston have signed onto the opposition effort, which is playing out at the Moakley federal courthouse in South Boston. The opposition effort also alleges Open Meeting Law violations, which councillors have a history of making.

The Wu administration has hired Lon Povich, a top lawyer who worked for Gov. Charlie Baker, and Jennifer Miller, who served as chief legal counsel for the state Senate, to defend the map.

Redistricting is a decennial city-level process that follows the US Census effort. This round was driven in large part by a boom in South Boston’s population, mostly in the neighborhood’s Seaport area. That meant District 2 had to shed population as Dorchester’s District 3 needed to gain it, leading to bitter fights and harsh words between councillors.

As the public housing developments and their hundreds of residents, many of them people of color, shifted to District 3, several predominantly white Irish areas of Dorchester, precincts in Cedar Grove and Adams Village, were moved to District 4, which is represented by Councillor Brian Worrell, who is Black.

Lynch, who has represented that area of Dorchester as part of his Congressional district, which runs from South Boston down to Brockton and Bridgewater, said Cedar Grove and Adams Village have veterans posts and youth sports leagues that tie them to Dorchester’s Neponset neighborhood. “The churches are what holds it all together though,” he said. “It’s been that way for a long, long time.”

Asked what the redrawn map does to the area, Lynch said, “It changes the whole dynamic there. Very divisive, I think, in terms of the identity of the area.”

Lynch also criticized the map for moving the public housing developments out of District 2. “Did you try some other ways to balance out the population?” Lynch asked rhetorically, referring to councillors.

The councillors who oppose the enacted map, including Baker, appeared in court to take in Lynch’s testimony. The Reporter previously reported that Baker is backing the federal lawsuit with $10,000 from his campaign account.

Several City Council aides and former state Sen. Dianne Wilkerson, who worked on the map as part of a coalition of advocacy groups, also sat in the audience, as did Rasheed Walters, who has previously written columns for the Boston Herald and signed onto the lawsuit. Walters, whose Council district (No. 4) didn't change after the passage of the redistricting map, declined to speak with reporters.

Closing arguments in the federal case could come on Wednesday.

Moon Duchin, a Tufts University expert on redistricting who has testified across the country, took the stand after Lynch. A witness for defenders of the map, she said the map enacted by councillors resembles the one adopted 10 years ago, during the last redistricting process.

Almost 10,000 people were reassigned from District 2 to District 3, according to Duchin, who also offered advice to councillors when they were hashing out changes last fall.

Judge Patti Saris, who is hearing the arguments, noted that the “big grievance” is from the Neponset area and asked would the map be off-kilter if Neponset and the Little Saigon area of Fields Corner, which was moved into District 3 in the enacted map, were left alone.

Duchin said the lines had to be redrawn because of the population shifts in South Boston, and the ripple effect that follows.

When Saris noted that many in Neponset did not stay in the same council district, Duchin acknowledged that they were shifted. “But someone always has to be,” she said, noting that 58,000 people, the vast majority, remained in District 3.

She added there are millions of ways — a “vast universe” — that a City Council map could be redrawn. But what councillors finally sent to Mayor Wu’s desk was largely a “status quo” map, she said.

There wasn’t a Voting Rights Act violation within the map, she said, as some map opponents have claimed with respect to District 4. The opponents argued the enacted map dilutes the Black vote in District 4 by pulling in high-voting white precincts.

Asked if District 4 as it exists still offers Black voters an effective opportunity to elect candidates of their choice, Duchin said it does.