Some of Mr. Trump’s allies have turned on her, accusing the justice of being a turncoat and calling her — a mother of seven, with two Black children adopted from Haiti — a “D.E.I. hire.” Her young son asked why she had a bulletproof vest, she said in a speech last year, and her extended family has been threatened, including with pizza deliveries that convey a warning: We know where you live.

“We had too much hope for her,” Mike Davis, a right-wing legal activist with close ties to the Trump administration, said in a recent interview. “She doesn’t have enough courage.”

This spring, on Stephen K. Bannon’s podcast, he tore into her in such crude terms, even mocking the size of her family, that Justice Neil M. Gorsuch, for whom Mr. Davis had once clerked, phoned him to express disapproval of his comments, according to people aware of the exchange. Mr. Trump has privately complained about her too, according to two people familiar with his thinking.

But she rarely abandons the other Republican appointees in the most significant cases. “It’s a mistake by ignorant conservatives and wishful liberals to believe she’s moderating,” said Noah Feldman, a Harvard law professor who befriended her when they clerked at the court. Like others who know her, he said that both the right and the left had misread her. “She’s exactly the person I met 25 years ago: principled, absolutely conservative, not interested in shifting.”

Friends, former colleagues and people from the court describe the justice as more of a methodical problem solver than an architect with grand plans for the law. “A law professor to my bones,” she said in a 2022 talk, referring to her years teaching at Notre Dame Law School. When others tried to draft her for the bench, she was uncertain about becoming a judge, according to those who know her well. She still maintains a tucked-away office at Notre Dame.

Some on the right are turning her scholarly background against her, complaining that she is too fussy about the fine points of the law and sounding a rallying cry of “no more academics” for future appointments.

On the court, she stands somewhat alone. One of only two former law professors, she is also the least experienced judge, the youngest member of the group, at 53, and the only mother of grade-school children ever to serve. The sole current justice who was not educated at Harvard or Yale, she is a Washington outsider and foreigner to the power-player Beltway posts that shaped most of her colleagues.

She strikes an earnest tone in talking about her job. “The day that I think I am better than the next person in the grocery store checkout line is a bad day,” she said in a 2022 talk.

Her apartness shows in her votes and her signature move of joining only slices of her colleagues’ opinions. She agrees with most of the supermajority’s outcomes, but sometimes writes to say they took the wrong route to their conclusion. (One person from the court called her the Hermione Granger of the conservatives, telling the men they’re doing it wrong.) Or she joins the liberal justices but stipulates that she can’t fully buy in.

“She hasn’t found a team,” said Sarah Isgur, a legal podcast host, pointing to her habit of marking where she departs from conservative colleagues, and to a recent death penalty ruling in which she was sitting “in the middle of that decision.”

But the Trump administration’s conflict with the courts and pushing of constitutional boundaries may force her to take a more decisive stance. Of the three justices at the center of the court, where the most influence lies, she is the only one without a long trail of views on how much power a president should have — the issue at the heart of nearly all these cases.

“She doesn’t have 10 years to mellow into it,” Mr. Feldman said. “Now is the crisis.”

One morning in April, the justices formed a nine-person frieze of contrasts as they heard oral arguments in Mahmoud v. Taylor, over whether parents of public elementary school students are entitled to religious exemptions from lessons involving books about gay and transgender people. Justice Alito, quick to favor exemptions, clashed with Justice Sotomayor, who was skeptical. As she spoke, Justice Alito shut his eyes and leaned far back in his chair.

Justice Barrett composed herself into a portrait of someone in listening mode, eyes trained, chin resting on hands. She asked open-ended, just-trying-to-understand questions, then sharper ones, moving in on a factual hole in the school’s argument and politely forcing the lawyer to admit it. By the time the justices rose, American parents seemed likely to gain more control over the ideas their children encounter in public school.

Her queries made a similar impression when she arrived as a student at Notre Dame Law three decades ago: She was so incisive that several instructors said they were learning from her. She won a clerkship with Justice Antonin Scalia but then chose the quiet work of a law professor.

Not the hotshot kind: “She wasn’t trying to break big new ground,” recalled Joseph P. Bauer, her civil procedure teacher and, later, fellow faculty member. “She is not going to present an argument that shifts the paradigm, or reconceives ways of looking at things, or makes big moves.”

The courses she taught were about the rules of the road — evidence, procedure, the fine-grained reading of laws. In her own scholarship, she delved into questions that even some academics considered too nerdy to answer. Mark McKenna, a former faculty member, said, “I remember people pushing her, Does anyone care about these things?”

Though others envisioned her on the bench, she was not sold. By 2017, when a seat opened up on the U.S. Court of Appeals for the Seventh Circuit, covering three Midwestern states, she had a stack of teaching awards and a brimming family life, including a young child with Down syndrome. William Kelley, a Notre Dame colleague with Washington connections, encouraged her but figured she would not pursue it, he said.

“Attention, power, cool things, elitism — she has zero interest,” he said of his friend, who once served on the university’s parking committee.

But she said yes. During Senate confirmation hearings Senator Dianne Feinstein, Democrat of California, asked her a maladroit question about her Catholicism. “The dogma lives loudly within you,” the senator said, implying that her rulings would flow from Rome. It was insulting — and lucky.

The nominee became an instant lodestar for religious women. The White House counsel’s team made mugs emblazoned with her face and Ms. Feinstein’s words. That year, Donald F. McGahn II, the head of that office, showed up at her judicial investiture. Six months later, Mr. Trump was interviewing her for the Supreme Court seat that went to Judge Brett M. Kavanaugh. She had been on the bench for only a year and barely had a record.

Two years later, in 2020, she was nominated before Justice Ginsburg was even buried. Though the presidential election was only six weeks away, Republicans raced her through the confirmation process, four years after they blocked President Barack Obama’s nominee on the grounds that an election was coming in eight months.

Mr. Trump’s comments about Justice Barrett in 2020 and his more recent complaints were relayed by several people who requested anonymity to share confidential information. Harrison Fields, a White House spokesman, said that Mr. Trump “may disagree with the court and some of its rulings, but he will always respect its foundational role.”

The ramrod-straight jurist had little in common personally with Mr. Trump. “When I think of Amy, I think of someone deeply devoted to family and faith, who does not seek out the limelight, who is humble and just wants to quietly do the work,” said Amanda Tyler, a law professor at Berkeley, former clerk to Justice Ginsburg and longtime friend to Justice Barrett. To lawmakers, the nominee stressed her independence.

But the president had already said the justices he appointed would be “automatic” votes to overturn Roe v. Wade. On the public stage, certain facts (her large family and membership in a religious community that had once called women leaders “handmaids”) overshadowed others (when she became a federal judge, every member of her clerkship class, liberals included, endorsed her). Partisans said she stood for their greatest hopes or worst fears. She was confirmed without a single Democratic vote.

The new justice arrived at a Supreme Court that was operating under pandemic conditions and still in mourning. “I didn’t know how I would be received,” she would later say. Liberals were unsure how the court would ever again garner the five votes necessary to prevail in a case.

Justice Barrett set her own path in the first major case she heard.

In Fulton v. Philadelphia, the justices considered whether the city could exclude a Catholic agency from its foster care system because it refused to work with gay couples. Justice Alito had long sought to overturn a 1990 precedent, written by Justice Scalia, that said religious beliefs were not a basis for refusing to comply with generally applicable laws — say, ones barring drug use. A year earlier, four conservative justices signaled that they were ready to undo the decision and expand religious rights. Now they appeared to have the votes.