At The Original Goodfella’s, a well-known Staten Island pizzeria where photographs of Republican politicians are prominently displayed, the news sank in painfully: This borough, a rare conservative outpost of New York City, was being tossed into a congressional district with the liberal residents of Park Slope, Brooklyn.

“Park Slope is more of a younger crowd with yuppies, hipsters,” said Carlo D’Angelo, 28, a Trump supporter who, when asked about who won the 2020 presidential election, said, “Only the man in the sky, only God, knows.”

Staten Island was more “family-oriented and traditional,” he added, speaking near a framed display of a fork that ex-mayor Bill de Blasio, a Park Slope resident, scandalously used to eat pizza. “It’s two different, completely different, viewpoints.”

The feeling was mutual outside the Park Slope Food Coop, the famously liberal Brooklyn grocery where social consciousness pervades every aisle, in a neighborhood that is home to many left-leaning families. Pamela Plunkett, 57, stood nearby, across the street from a meditation center, as she questioned how the wildly divergent politics and needs of residents in the new district would work.

“I hate to say it, they’re one of the five boroughs, but it’s almost like they’re an outlier,” she said of Staten Island, noting differences in attitudes around issues including politics and the pandemic. “That’s why I’m worried about being grouped in with them.”

The once-in-a-decade redistricting effort has created unusual congressional district lines all over the country, reflecting a partisan process embraced by Republicans and Democrats alike. But perhaps no other district in New York City contains constituencies so clearly in opposition to each other as the reconstituted 11th, whose new lines are expected to better position the Democratic Party to seize a seat now held by Representative Nicole Malliotakis, the lone Republican in the New York City delegation.

On Staten Island, the occasional “Thin Blue Line” flag in support of law enforcement flutters in spacious front yards of single-family homes, while in dense brownstone Brooklyn, “Black Lives Matter” signs have often dotted windows, reflecting national debates over both crime and police brutality. Voters on either side of the Verrazzano-Narrows Bridge are often vocal about their political identities — but many liberal Brooklynites joined marches to protest the Trump presidency, while conservative Staten Islanders embraced him early, even with other Republicans in the running in 2016.

“They put two communities together that have literally nothing in common other than they happen to all live in the same city,” said City Councilman David Carr, a Staten Island Republican. “In terms of values, in terms of interests, they couldn’t be further apart. And they’ve created a district that’s going to be permanently at war with itself.”

The new lines reflect an aggressive reconfiguration of the state’s congressional districts led by Democratic lawmakers, creating clearer opportunities to flip several House seats in this year’s midterm campaigns, as Democrats strain to maintain their congressional majority in a difficult political environment.

Before redistricting, the district was anchored in Staten Island and included parts of more conservative southern Brooklyn enclaves. Under the new lines, the district sweeps into many neighborhoods that are home to wealthy liberal voters and younger left-wing activists — though neither part of the district is monolithic: There are Staten Island Democrats and some Brooklyn conservatives, especially in the Bay Ridge area.

In 2020, the district supported Mr. Trump by about 10 percentage points. If the new district lines were in place for the 2020 election, the district would have backed President Biden by roughly the same margin, according to data compiled by the City University of New York.

Ms. Malliotakis said the new lines seemed aimed at “silencing the voices of the current district, and tilting the scale to give whoever the Democratic nominee is an advantage.”

The Staten Island Republican Party dubbed redistricting plans “cancel culture,” an effort to “subvert the voices of Staten Islanders by tying our borough to de Blasio’s Park Slope.”

Democrats have defended the congressional maps as fair, while Republicans have filed a lawsuit, which may face an uphill battle.

“Had we sought out people that voted the same way in order to keep them together, that would have been the definition of illegal gerrymandering,” said State Senator Michael Gianaris, a Democrat and leader of a task force that drew the lines.

“Maybe at the end of the day, this will have the effect of bringing people together,” he said.

That will be exceedingly difficult in the 11th, should the lines hold.

But whatever the evident governing difficulties, a fierce battle is unfolding to represent the district as Ms. Malliotakis, who has tied herself closely to Mr. Trump and voted against certifying the results of the 2020 election, runs for re-election. She also broke with her party to vote for the infrastructure bill.

While candidates in many races face difficult balancing acts between appealing to the most die-hard partisans in a primary and achieving broader appeal in a general election, those tensions will be thrown into sharp relief in the 11th District.

“It certainly gives the Democratic nominee a very good chance,” said John Mollenkopf, director of the Center for Urban Research at the Graduate Center of CUNY, of the new district lines. “But that’s going to take a Democratic nominee who can appeal to the more conservative Democrats on Staten Island.”

On the Democratic side, the biggest open question had been whether Mr. de Blasio would run, an idea that sparked viscerally negative reactions on Staten Island. At Grant City Tavern, where the ceiling is designed to look like an American flag and a portrait of Mr. Trump hangs on the wall, one patron knocked over his drink at the mention of the former mayor.

He commissioned polling around the race, but took himself out of contention on Tuesday.

The most high-profile Democrat in the race is former Representative Max Rose, who won the earlier configuration of the district in 2018 and lost to Ms. Malliotakis in 2020. Mr. Rose, who grew up in Park Slope, had cast himself as more of a Staten Island Democrat with a brash personal style and relatively centrist politics, and some party officials see him as a strong general election fit in the new district.

But some previous positions — he was one of the last Democrats to support an impeachment inquiry into Mr. Trump and has noted areas of agreement with him; he has criticized left-wing Democrats like Representative Alexandria Ocasio-Cortez and has rejected some sweeping left-leaning policy goals — may unnerve more progressive voters.

“Certainly I stand opposed to Medicare for All, I do not stand opposed to universal health care,” said Mr. Rose as he knocked on doors in Mariners Harbor, a more Democratic neighborhood where he earned a warm reception on a freezing recent evening. He still opposes the Green New Deal because of the scope of its ambitions, but “that doesn’t take away from the fact that I think we have got to get to a net-carbon neutral economy around 2035.”

After spending much of his last campaign distancing himself from the national party, he is also calibrating that messaging. He said the Democratic Party brand is “toxic” in some parts of the country but was not eager to dwell on that subject, pivoting to his hope to be “bold and nonetheless unifying” through a focus on issues like universal child care and other investments in working-class Americans. He also cast the upcoming elections as a test of “the very essence of our democracy.”

Brittany Ramos DeBarros, who like Mr. Rose served in the military, is running to his left. She is hoping to energize diverse constituencies to vote in unusually high numbers across the district, and wants to build on recent left-wing local wins in places like Sunset Park and Park Slope.

“We feel really excited about welcoming new neighborhoods that have also had incredible progressive victories recently with other bold women of color,” she said. “We need a bold leader who is focused on the lived experiences of everyday people across the district because it is so diverse.”

Some Democratic officials expect that liberal residents of the district will be motivated to flip it regardless of who the nominee is — and for some voters, perceptions of general election viability may be a factor in the primary, too.

“Especially in these times, I think electability really, really matters,” said Jocelyn Baker, 47, a midwife from Sunset Park, standing outside the food co-op in Park Slope. Still, she extended a hand to new fellow constituents. “As much as I love my bubble, I don’t think it’s helpful for us to just stay in our bubbles and not be exposed to anything else.”

On Staten Island, Mr. Carr, the councilman, took a far dimmer view of the district’s contours.

“There’s no way a congressman of either party is going to be liked by the other half of the district no matter what they do,” he said. “You now have a district where, as soon as that person’s in office, the other half of the district is preparing to try to unseat them.”