Nancy Mace’s District Moved Right. Then She Helped Oust McCarthy.

When South Carolina’s First Congressional District evoked wide sand beaches, Spanish moss, oyster and cocktail bars and hot yoga, its Republican congresswoman, Nancy Mace, made her name appealing for moderation on abortion, climate change and marijuana legalization, while calling out the G.O.P.’s biggest bomb throwers as bigoted clowns.

Then in 2022 came the redrawing of district lines, as rural reaches like Cordesville, S.C., with their modest one-story brick homes and prefabricated double-wides, replaced the graceful mansions and Black neighborhoods of Charleston. So last week, when Ms. Mace shocked Washington and joined seven hard-core conservatives to oust Representative Kevin McCarthy from the speaker’s chair, her new constituents were not surprised.

“I’ve always heard the squeaky wheel gets the oil, and when you’re a female, you don’t get heard unless you’re loud,” said Janet Jurosko, a new constituent of Ms. Mace’s from Cordesville and the auditor of Berkeley County, S.C., which joined the First District in its totality last year. “I think she’s doing a good job — I really do.”

Ms. Mace still calls herself an iconoclast, but her transformation from denouncing the likes of Representative Matt Gaetz, Republican of Florida, to joining him in the first overthrow of a sitting speaker underscores a truism: Voters lead their politicians; politicians don’t lead their voters.

Though Ms. Mace’s turn to the MAGA wing of the G.O.P. has been ongoing, the increasingly red nature of her district may help explain her latest move. She weathered a Republican primary challenge from the right in 2022 from candidate endorsed by former President Donald J. Trump and learned the lesson that criticizing or opposing Mr. Trump in the G.O.P. would always be a trial.

But voters in her district believe the new map charted her course.

“Nancy has always been and will always be a maverick,” said Josh Whitley, a Berkeley County commissioner and a Mace ally. “But she has also always been very mindful of her constituents.”

The way South Carolina’s First Congressional District was redrawn by the Republican-led legislature touches on two consequential effects of gerrymandering at once: political dysfunction and polarization, and the potential for Black disenfranchisement. As Ms. Mace helps choose a new speaker in the Capitol on Wednesday, her district’s map will be the subject of oral arguments before the Supreme Court where the conservative supermajority has recently shown sensitivity to the issue of racial gerrymandering.

Fresh off ruling that the Alabama Legislature had unlawfully diluted the strength of Black voters and ordering that congressional maps be redrawn — and rebuking G.O.P. legislators’ second attempt — the court will decide, most likely by January, whether South Carolina’s reworked First District constitutes an illegal racial gerrymander. The current House map moved 62 percent of the Black voters in Charleston County — 30,000 of them — to the Sixth District, a seat that Representative James Clyburn, a Black Democrat, has held for three decades, and moved inland white voters like Ms. Jurosko into the First District.

In 2020, she beat an incumbent Democrat, Joe Cunningham, by a single percentage point to win the seat. With the new map, Ms. Mace won re-election in 2022 by 14 points.

“We were already able to convince three judges that this was a racial and not a partisan gerrymander,” said Antonio L. Ingram II, an assistant counsel at the NAACP Legal Defense Fund and a litigator on the South Carolina case. “We’re confident the Supreme Court will agree.”

Ms. Mace declined to comment for this article. She has ascribed her shifting allegiances toward the G.O.P.’s right-wing rebels to a natural independent streak.

“I’ve had my ups and downs with a lot of members in Congress, because as an independent voice, I will call the balls and strikes regardless of the consequences,” she said Sunday on CBS’s “Face the Nation.”

Ms. Mace’s rightward shift reflects the broader politics of the country, which is tearing apart along partisan lines, driven by the self-sorting of voters into Democratic and Republican districts and states, and by politicians drawing district maps that make House members far more wary of challenges from their own party than of defeat by the opposition party.

A new analysis by the nonpartisan Cook Political Report confirmed the trend. Today, just 82 of 435 House districts fall within an expected lean of five percentage points Republican or five percentage points Democratic, the report’s definition of a swing district. That is exactly half of the 164 districts that were considered up for grabs in 1999. The number of heavily Republican seats jumped over that time from 150 to 189, while the number of heavily Democratic seats rose from 121 to 164.

The number of “hyper-swing” seats — between a lean of three percentage points either way — has declined to 45 from 107 in 1999.

David Wasserman, the analyst who wrote the report, pointed a finger. “Republican-controlled states account for a majority of the swing seat collapse,” he wrote.

The voluntary grouping of like-minded voters had an important effect, said Julie A. Wronski, a political scientist at the University of Mississippi who specializes in polarization.

Both groups also marinate in their own chosen media outlets, as local news atrophies and politics become nationalized along distinct battle lines, she added.

But there is a human cause to the hardening partisan divide. Mr. Wasserman found a 70 percent drop since 1997 in the number of swing seats in states where redistricting was in Republican control, from 56 to 17, while nonpartisan or bipartisan redistricting commissions in nine states — many of them leaning Democratic, like Colorado, California, Hawaii, Michigan, New Jersey and Washington — have kept the decline less precipitous, from 41 to 25.

Beyond partisan gerrymandering, the Rev. Nelson B. Rivers III, a longtime civil rights activist in North Charleston, said racial gerrymanders leave the most vulnerable communities ignored by representatives who no longer need their votes for re-election. The new map separates North Charleston and most of Charleston from the South Carolina coast and the shared issues of shipping, climate change and fisheries, binding them instead to inland Columbia and state capital issues far removed from the Lowcountry.

“It was done to not have two Black members or two people who would fight for the interests of the Black community and the poor community and the gay community and all the communities that need somebody in Congress to fight for them,” he said.

Black voters still in Ms. Mace’s district — just 17 percent of the voting-age population, the lowest in the state — agree.

“We continue to lose our voice, we continue to lose representation, and we need that,” maintained Taiwan Scott, 47, a Black activist and real estate agent in Hilton Head Island, S.C., who is a plaintiff in the case before the Supreme Court. A member of the Gullah Geechee community, a distinct cultural group of Black Americans on the Southeast coast, recognized for its dialect and lifestyle, Mr. Scott pointed to his family cemetery, now on the 18th hole of a golf course in Harbour Town.

“I don’t care if that’s a Republican or a Democrat,” he said. “My concern is our voice.”

In 2020, Ms. Mace was a prized Republican recruit, the first female graduate of the Citadel, South Carolina’s military college.

In Washington, she said in her first speech on the House floor that the House needed to “hold the president accountable” for the Jan. 6, 2021, Capitol attack. She traded barbs with some of the most prominent of Mr. Trump’s allies in the House Republican Conference, calling out Representative Lauren Boebert of Colorado for anti-Muslim comments. She memorably used a series of emojis — a bat, a pile of excrement and a crazy clown — to describe Representative Marjorie Taylor Greene of Georgia and clashed with Mr. Gaetz, who amplified an attack by the right-wing provocateur Jack Posobiec denouncing Ms. Mace as a “scam artist” for promoting coronavirus vaccinations.

Though Mr. Trump endorsed a Republican challenger, Katie Arrington, against her last year, Ms. Mace prevailed.

The redrawing of South Carolina’s First District did not just add Republican voters. It diluted the power of affluent, genteel “country club” Republicans and transplanted northern retirees from coastal towns like Isle of Palms, Sullivan’s Island, Kiawah Island and Hilton Head. The district didn’t just become more Republican. It became more Trumpist.

After her landslide re-election, she has found a fiercely partisan voice within the Republican-led House driving the impeachment inquiry of President Biden with unsupported accusations of broad familial corruption. She claimed to have suffered long-term ailments from her Covid vaccination and has waded into transgender issues, for instance snapping at hearing witnesses for using “woke” language like “pregnant people.”

The day after she joined rebels to oust Mr. McCarthy, Ms. Mace found herself in the 19th-century brick townhouse of Stephen K. Bannon, the former Trump adviser who was convicted last year of contempt of Congress, for a joint appearance on Mr. Bannon’s podcast with her former antagonizer, Mr. Gaetz.

Her inland voters are fine with the new Nancy.

“It’s politics — hello,” Jean Moureau, 68, said as he stood outside Howard’s Restaurant in Moncks Corner, in North Berkeley County. “We’ll be on whatever side our bread is buttered.”

His lunch mate, Sue Stevenson, 70, liked what she was seeing in Ms. Mace, “absolutely.”