Frustration over decades of underrepresentation of minorities in Islip Town government — and the divide that some residents say is geographic, economic and political — has been simmering for years.

That dismay forms much of the foundation for a federal lawsuit alleging the town’s at-large voting system, in which candidates are selected by a townwide majority rather than localized districts, gives white residents more influence over who is elected.

The lawsuit — filed in June in U.S. District Court in Brooklyn by four Latino Brentwood residents and two advocacy groups — said the town’s voting system leads to “inequitable treatment of Islip’s minority Latino community” and denies them “adequate electoral representation.”

While minorities make up about half of Islip’s population, according to census figures,  none has ever been on the town board.

The plaintiffs are seeking to change the town’s voting system to district elections so that Latinos and people of color are the majority in at least one ward. The Latino community then would have an equal opportunity to elect a candidate who will be responsive to the needs and concerns of minority residents, the lawsuit said.

“Every day I see the ways Latino neighborhoods are underserved and underrepresented,” said plaintiff Magali Roman, 60, a nonprofit program coordinator. “The town handles complaints from our neighborhoods differently. These problems start out small and when the town neglects them, they turn into bigger problems.”

Islip Supervisor Angie Carpenter declined to comment on allegations of neglect.

“It’s the town’s policy not to comment on pending litigation,” Islip spokeswoman Caroline Smith said.

The town must respond to the lawsuit by Oct. 10. The Suffolk County Board of Elections, also named as a defendant, is seeking to dismiss the case and has denied knowledge of the allegations, including that the town’s electoral system prevents minorities from being elected, according to court records.

Islip Councilman James O’Connor, of Great River, said the town board “is responding evenly” to all residents.

“I represent all communities evenly. I don’t see a dividing line,” O’Connor said. “We’re trying our best.”

The lawsuit says the town’s socioeconomic statistics show that Latino residents face the “brunt of discrimination” in employment, education and policing compared with white residents.

In communities north of Sunrise Highway, Brentwood and Central Islip, respectively, have median incomes of $69,457 and $66,467, according to U.S. Census estimates. Brentwood’s population is 68.5 percent Latino and 16.4 percent black, while Central Islip’s population is 52.1 percent Latino and 25 percent black, according to the 2010 census. In Bay Shore, the median household income is $66,956 and it is 30.8 percent Hispanic and 19.6 percent black, according to census estimates.

South of Sunrise Highway, hamlets such as Islip and East Islip have white majorities, with Latinos and African Americans composing respectively 18.5 and 7.2 percent of the population, census figures show. Those hamlets have median household incomes of $97,344 and $116,487, respectively. Savyille is less than 5 percent Latino and about 1 percent African American. Its median income is $103,243.

Islip is Long Island’s third-largest town, with a population of 335,000, and its median income is $87,066, statistics show.

Preservationists have long called the area south of Sunrise Highway Long Island’s original Gold Coast because wealthy families such as the Vanderbilts once owned expansive estates there. Waterfront homes still dot the shoreline on both the mainland and on Fire Island. North of Sunrise, illegal dumping and gang violence have made national headlines in communities such as Brentwood and Central Islip.

Only three town board members have come from the north side of town in the past 25 years, and none has come from Brentwood, the most populous hamlet, election records show. Islip has only elected one town official of color: former town clerk Joan Johnson, an African-American woman who served from 1991 to 2007. Johnson did not respond to requests for comment.

“It’s a tale of two towns,” said Renee Ortiz, a Democrat who ran unsuccessfully for Islip Town Board in 2011 as the first council candidate of color to be nominated by a major party. She is not a party in the lawsuit. “It’s literally divided — divided by Sunrise Highway.”

Islip is one of dozens of municipalities across the country — including some on Long Island — that have been sued over allegations their at-large election systems violate the federal Voting Rights Act. Lawsuits in similar cases forced municipalities in New York, Texas, California, Georgia, South Carolina and more to create council districts.

Hempstead, Brookhaven and North Hempstead already have council districts. Hempstead began voting by district in 2000 after a 12-year legal battle that ended with a U.S. Supreme Court decision that upheld the U.S. District Court’s ruling that at-large elections discriminated against black residents. The voting system in Brookhaven and North Hempstead changed by referendum in 2002 and 2003, respectively. A judge sided with Babylon Town in a similar lawsuit in 1996, and residents are considering a referendum on the issue.

Islip residents debated creating council districts for years and voted against them in a 2006 referendum. Opponents said they were concerned about having only one board member to represent them instead of five. The five council seats include the town supervisor.

Republican Legis. Steve Flotteron, who served on the town board when the issue went to referendum, said ward districts could make for a “parochial” form of government, with representatives “fighting for their share” instead of looking at “the whole health of the town.”

“I want them all to be accountable for the decisions in my own backyard,” Flotteron said.

Opponents said they are concerned that one district would have more representation because the supervisor would live in the same ward as one council member. Operating costs could also increase if council members decide to open district offices with their own staff, they said.

Hempstead was the first Long Island town to switch to council districts following a lawsuit led by Hempstead-based attorney Frederick K. Brewington and plaintiff Dorothy Goosby, who is now Hempstead Town’s senior councilwoman. Brewington also represents the Islip plaintiffs.

Goosby said Islip’s northern residents are experiencing a lot of the same problems black Hempstead residents had before council districts were created, such as inadequate parks and roads.

“We were not getting our share of services,” she said. “We didn’t have anyone to fight for us. It was very necessary just to get the basic things we need.”

Hempstead residents now know whom to turn to — their district representatives — when they have concerns in the community, from trash on streets to road conditions, Goosby said. Parks and roads have improved, and more African Americans have since been elected and promoted within town government, she said.

The U.S. District Court approved six districts in Hempstead after the municipality sought to create two districts: one essentially for white residents and one for black residents, Brewington and Goosby said.

Being able to elect a candidate of the Islip Latino community’s choice would “allow its voice to be fully and properly heard,” Brewington said.

Islip council districts would change the status quo of neglect in areas such as Brentwood, Central Islip and North Bay Shore, which have “crumbling roads” and “dark streets,” the lawsuit said. Town officials have failed to respond to resident complaints, including concerns about MS-13 gang violence, the lawsuit said.

Velene Gallagher, executive director of the Central Islip Civic Council, said town officials have made “an effort to improve the north side” of town, noting that the hamlet’s roads have been paved and abandoned houses have been cleaned up in recent years. If there is a difference in treatment by hamlet, “I don’t think it’s malicious,” she said.

“I just think it’s been done that way forever,” Gallagher said.

Plaintiffs said the illegal dumping in Roberto Clemente Park in Brentwood in 2013 and 2014 is an example of why minority representation on the council is important.

The dumping of nearly 40,000 tons of contaminated debris closed the park, including its pool, for three years. The dumping, which affected three other properties, led to the convictions of two former town park officials and three other men on charges such as endangering the public health, safety or the environment. The pool reopened in August.

María Magdalena Hernández, a 53-year-old receptionist and plaintiff, said a district representative would be more likely to provide information in Spanish, the language spoken by Hernández and 25 percent of town residents, according to census estimates. She noted that town officials did not bring a Spanish translator to the only public meeting about the dumping at Clemente Park.

Lee Koppelman, a Stony Brook University professor who led the Long Island Regional Planning Board for 40 years, said officials who are elected at-large “could ignore the minority communities” because they need support only from the majority of voters to get into and stay in office.

“It’s the advent of councilmanic districts that give both ethnic minorities, as well as women, the chance to get into government,” he said.