D.A. King, the head of an Atlanta-area group that opposes illegal immigration, heard word of Justice Anthony M. Kennedy’s retirement while vacationing on St. Simons Island, off the Georgia coast. He was elated thinking of the conservative who might replace him.

Kristen Clarke, a civil rights lawyer, heard the news on the radio not far from the Supreme Court itself, as she was driving to a Capitol Hill hearing about the Voting Rights Act. She figured her job defending voting rights was about to become much more of a challenge.

In West Hollywood, Curtis Collins was working out at Barry’s Bootcamp, and he said the Supreme Court justice’s announcement dominated the Wednesday afternoon conversation among the predominantly gay group of men exercising there. “Everybody was talking about it, how appalling it was,” he said. “Everyone was saying they were scared. We don’t normally talk about politics in there.”

And in North Carolina, as the news of the impending retirement flashed on Amy Mahle’s phone, she wondered whether God might soon answer her prayers — and let her finally see the high court overturn Roe v. Wade, the landmark 1973 case establishing a constitutional right to abortion. “I think it’s possible,” she said. “I would love that.”

It is not exactly shocking when an 81-year-old man decides to retire after 30 years on the job. And yet, Justice Kennedy’s announcement this week still managed to deliver a powerful jolt to the nation. His departure comes at a fragile time for the country, as a first-term president has vigorously assumed the role of disrupter-in-chief. President Trump said he wanted to pick a jurist who could serve at least 40 years on the court, potentially cementing the president’s impact on the country for generations.

Justice Kennedy, a centrist swing vote, is likely to be replaced by a reliable conservative, tipping the institution decidedly rightward. For many conservatives, this amounts to a kind of judicial grand prize, one that outweighs any concerns about Mr. Trump’s departures from conservative orthodoxy.

For many liberals, the departure was almost too much to bear, particularly after a month in which they were disappointed by Supreme Court rulings that, among other things, narrowly upheld Mr. Trump’s travel ban, curtailed union power, and let stand a plan to purge state voter rolls in Ohio.

“We are so much more screwed today than we were yesterday, and we were pretty screwed yesterday,” said Monica Russo, a 41-year-old stay at home mother on Long Island, who had just put her daughter down for a nap when she saw the news on Twitter.

Coming on the heels of Tuesday’s decision upholding the travel ban, Ms. Russo felt overwhelmed.

“My stomach dropped,” she said. “I know that Trump is hellbent on replacing any justice with somebody who would be a threat to reproductive rights. It makes me want to do anything that I can to make sure that everybody knows the importance of voting in November.”

Americans of all political persuasions are now bracing for what is likely to be an incendiary confirmation battle, and pondering what effect a newly constituted court will have on longstanding issues, like abortion, and more recent controversies, like immigration, that Mr. Trump has stoked.

Mr. King, eagerly awaiting who will move into Mr. Kennedy’s office, is a well-known and influential activist in Georgia whose flavor of anti-illegal immigrant activism prefigured that of Mr. Trump: He is president of a group called The Dustin Inman Society, named for a teenage boy killed in 2000 in a wreck with an undocumented immigrant driver.

“I trust that the president is going to nominate a pro-borders, pro-enforcement, pro-American Supreme Court justice that will recognize the importance of immigration,” Mr. King said. “There is no universal right to live in the United States.”

His sentiments stood in stark contrast to those of Cindy Nava, an immigrant from northern Mexico who was brought to the United States as a child by her parents. “There was a point where we had faith in our court system in protecting immigrants,” she said Wednesday. “But I feel like everything is in jeopardy. I’m sort of speechless at the moment.”

Ms. Nava, who now lives in Albuquerque and runs a nonprofit aimed at enhancing education opportunities for children, said the fear undocumented immigrants live in extends even to people who, like her, have recently become legal residents.

“There’s a growing sense that no one is safe unless you obtain citizenship,” she said. “Will the Supreme Court protect people like me? I just do not know.”

Mr. Trump may have once called himself “very pro-choice,” but in the lead-up to the 2016 general election, he promised to appoint justices who would overturn Roe v. Wade.

Still, the idea that the abortion rights case would ever fall had long seemed like a distant possibility to Ms. Mahle, 47, a program director at a North Carolina community college. A devoted Christian, she said that in 1998, she and her husband adopted a 12-year-old boy who she is thankful was not aborted when his biological mother became pregnant as a teenager.

“I really believe that the Supreme Court probably plays the singular most important role in the entire country,” she said. “They are lifetime appointees. They have the responsibility to interpret the Constitution. And I believe that they have a responsibility also to have a moral and ethical compass to lead our country according to the Constitution, and not be swayed by political agendas or societal trends.”

Diane Derzis, the owner of the sole abortion clinic in Mississippi, anticipated the next court veering away from Roe. “There’s no question,” Ms. Derzis said, her voice full of frustration. Abortion would be severely restricted “within a year,” she predicted.

“And how many well-educated people have looked me in the eye and said that that cannot happen? Well, now our No. 5 is leaving,” she said, referring to Justice Kennedy, who earned a reputation as the court’s “firewall” for abortion rights.

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Ms. Derzis, 64, had an abortion in Alabama as a 20-year-old married woman, one year after Roe was decided. “I just knew that I wanted more than to have a baby and be stuck,” she said. “I knew that I wanted more out of my life and I wasn’t ready to become a parent.”

Earlier this year, Gov. Phil Bryant of Mississippi, a Republican, signed into law a measure that would ban almost all abortions after 15 weeks of pregnancy, but Ms. Derzis’s clinic sued in federal court and blocked the law’s implementation.

Justice Kennedy, who was appointed by President Ronald Reagan in 1988, has disappointed liberals over the years with his decisions as much as he has heartened them. But there is a sense on the left that things are about to change, and for the worse.

With Kennedy’s Retirement, the Supreme Court Loses Its Center

The swing vote in many decisions, Justice Anthony M. Kennedy shifted toward more conservative opinions in the final months of his 30-year justice tenure.

“That swing vote has been critical in so many civil rights cases,” said Ms. Clarke, the director of the Lawyers’ Committee for Civil Rights Under Law. “These have been some devastating moments for people who care about the most vulnerable in the country. These last few weeks of the court are just a reminder of how high the stakes are.”

Though liberals see Mr. Kennedy’s record on protections for ethnic and religious minorities in a mixed light, they have almost universally applauded his legacy on gay rights. His official opinions on high-profile cases like Lawrence v. Texas in 2003 and Obergefell v. Hodges in 2015, to name just a few, are fundamental pillars in the framework of legal protections afforded to lesbian, gay, bisexual and transgender people in the United States today.

“Millions of L.G.B.T. people have come out of the closet and they are able to pursue dreams they weren’t to pursue a decade ago,” said Camilla Taylor, the director of constitutional litigation at Lambda Legal. “And by leaving at this time, he puts that legacy in jeopardy.””

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“His words about the equal dignity of lesbian gay and bisexual people may mean a great deal less now,” Ms. Taylor added. “Given the track record for President Trump in nominating ideologues, we’re extremely worried. Extremely worried.”

But in the divided nation there was also extreme relief. Paul Donahue, 53, is a financial adviser and part-owner of the Centennial Gun Club outside of Denver. He is a former mayor of Castle Rock, Colo., and has been a vocal supporter of gun rights.

On Wednesday afternoon, Mr. Donahue said that he was thrilled to hear that the president would have a chance to appoint another justice, and that he was looking for “someone who is more of constitutionalist” to fill the slot.

But it wasn’t just gun rights he wanted the next justice to protect — it was everything he saw as under threat by justices appointed in the Obama administration who have ended up “ruling in accordance with their feelings or emotions.” Those rights, he said, include freedom of speech and freedom of religion.

“I am forever grateful that President Trump is in that position and not President Clinton,” he said, imagining the difference the nation would be in if the last presidential election had resulted in a victory for Hillary Clinton. “I see the Supreme Court as the last hope, and I think that’s probably the best way to put it.”

Monica Davey, Elizabeth Dias, John Eligon, Simon Romero and Julie Turkewitz contributed reporting.