Justice Samuel A. Alito Jr. is the subject of intense speculation about whether he will retire in the coming months and give President Trump a fourth nominee.

Late last month, more than a hundred people gathered at the National Constitution Center in Philadelphia for a formal dinner featuring Justice Samuel A. Alito Jr.

The gathering of the justice’s former law clerks, academics and an array of conservative federal judges considered possible contenders to succeed him, was intended to mark Justice Alito’s 20 years at the Supreme Court, a milestone he reached this year.

But, according to multiple attendees, there was one topic none of Justice Alito’s fans and acolytes raised publicly: whether the justice will serve a 21st year.

Ever since President Trump returned to the White House, speculation has swirled about whether he will have an opportunity to name a fourth or even a fifth justice to the bench, which already includes three justices nominated by Mr. Trump during his first term. Those questions have mounted in recent months, as Mr. Trump’s political fortunes have waned and the chance that Democrats might win the Senate in November has grown.

Justice Clarence Thomas, who at 77 is the court’s oldest current justice, has over the years ruled out retiring, indicating he intends to serve for decades more. Justice Thomas could set the record as the longest-serving justice in the Supreme Court’s history if he stays through spring 2028.

As a result, the spotlight has been trained on the court’s second-oldest member, Justice Alito, who turned 76 this month.

The speculation has surged since Justice Alito proved unable to address the crowd of well-wishers in Philadelphia, falling ill during the dinner and being taken to a hospital, where he was treated for dehydration, the court said. He was back on the bench the following Monday, and was one of the most active questioners during the high-profile arguments this month over the Trump administration’s effort to limit the guarantee of birthright citizenship.

In interviews, Justice Alito’s friends, former colleagues and law clerks said that the justice is well aware of the political calendar and would prefer to have a Republican president choose his successor.

But even those in close contact with the justice said they are unsure about his plans. Speaking on the sensitive topic on the condition of anonymity, they said the taciturn justice has not indicated even to friends whether or when he might retire. They said they believe it is possible he has not yet made a firm decision.

Justice Alito did not respond to a request to discuss his plans.

The day after Mr. Trump was re-elected in 2024, Ed Whelan, a prominent conservative legal commentator, predicted that Justice Alito would retire in the spring of 2025, and that Justice Thomas would follow this year, giving the president a chance to lock in a conservative supermajority for years.

Mr. Whelan said in an interview this week that his predictions prompted strong pushback from close allies of the justices, and he would now be surprised if there is a vacancy this year. For Justice Alito, he said, there are so many big cases at the court and he has “the potential to be in the majority in a way that he couldn’t count on in his first dozen years” on the bench.

A nominee of President George W. Bush, Justice Alito joined the court in 2005. He replaced the moderate Justice Sandra Day O’Connor, making Justice Anthony M. Kennedy a critical swing vote in cases involving gay rights, abortion and the death penalty.

After Justice Kennedy’s retirement, Justice Alito helped move the court steadily to the right. In 2022, he wrote the majority opinion in Dobbs v. Jackson Women’s Health Organization, which overturned Roe v. Wade and eliminated the nationwide right to abortion. This term, the court is confronting a series of cases testing the legality of Mr. Trump’s policies and efforts to expand presidential power.

“I wouldn’t be surprised if he finds this a most interesting time to be there,” Mr. Whelan said. “So why leave now?”

A big potential reason is the election calendar. To varying degrees, justices want to be succeeded by someone who shares their judicial philosophy and ideology, and they consider a host of factors, including who will name their successor and the makeup of the Senate that would be asked to confirm the pick.

Months before his retirement announcement, Justice Stephen G. Breyer said he was struggling to decide when to hang up his robe.

“There are many things that go into a retirement decision,” he said in a 2021 interview with The New York Times, in which he recalled that Justice Antonin Scalia had said he didn’t “want somebody appointed who will just reverse everything I’ve done for the last 25 years.”

If Democrats took control of the Senate after the elections this November, Mr. Trump would most likely have a difficult time getting a nominee confirmed before the end of his term. And if a Democrat were elected to succeed Mr. Trump as president, Justice Alito would be in his 80s before there would be any chance a Republican might be back in office to name his replacement.

Many observers of the court see a cautionary tale in Justice Ruth Bader Ginsburg’s decision not to retire before the 2016 election, when President Obama could have named the liberal icon’s successor. Instead, President Trump nominated the conservative Justice Amy Coney Barrett after Justice Ginsburg died while in office in 2020.

All of those factors might propel a decision to leave soon, while Republicans hold 53 Senate seats.

But people close to Justice Alito warn a decision about when to retire is highly personal.

Retiring justices sometimes announce their retirements on the last day of the court’s term, in late June or early July, but there is no requirement that they do so.

The lack of clarity about Justice Alito’s plans has caused close watchers of the court to hunt for clues about his plans, examining his clerk hiring practices, the planned release of the justice’s first book and even comments from his wife, Martha Ann Alito.

As the end of the term approaches, Justice Alito has not yet hired a full complement of four law clerks, who traditionally start over the summer. But the justice is known for finalizing hiring later than his colleagues and is in the process of interviewing candidates, according to people familiar with his plans.

Justice Alito’s book, “So Ordered: An Originalist’s View of the Constitution, the Court and Our Country,” is scheduled for publication the first week in October, the day after the court’s new term begins. That timing could make it complicated for the justice to travel the country to promote his book should he remain at the court, hearing arguments and meeting in conference with his colleagues.

“It sure feels like Alito doesn’t plan on having a real job the Tuesday his book launches and instead thinks he’ll be free to run around the country promoting it,” the columnist Elie Mystal wrote in The Nation.

But the people who know Justice Alito well said they cannot imagine their introverted friend doing a major book tour. His publisher, Hachette Book Group, has not yet locked in fall promotional events and any events will depend on his schedule, according to a person familiar with the plans.

Others have suggested that his wife may be urging him to retire. Martha-Ann Alito has never been particularly fond of Washington after finding her husband’s confirmation hearings bruising.

In 2024, Martha-Ann Alito also indicated that she would have a greater ability to express her own opinions when her husband was “free of this nonsense,” comments captured in an unusual secret recording of the justice and his wife by a liberal documentary filmmaker at the Supreme Court Historical Society’s annual dinner. It is not clear if she was referring to his possible retirement.

Akhil Reed Amar, a Yale Law School professor who is friendly with the Alitos and attended the dinner in Philadelphia, said criticism leveled at the justice “may wear on him a bit.” (In 2024, The Times reported that flags that have been associated with 2020 election deniers were flown outside of Justice Alito’s residences, spurring significant scrutiny of the justice and his wife.) But Professor Amar said he had not asked Justice Alito about his plans.

“I hope he doesn’t, but he might,” he said. “I don’t want him to retire because I still think he has a lot to offer.”

Although there is no current vacancy, the last three presidents have had an opening to fill within their first two years in office.

A White House official declined to comment.

Robert Luther III, a professor at Antonin Scalia Law School who helped choose judicial nominees during Mr. Trump’s first administration said the White House Counsel’s Office is always preparing for the possibility.

“It’s a regular topic of conversation among the lawyers in the administration,” Mr. Luther said.

That conversation will take on more urgency, he said, as November and the midterm elections approach.

Liberal organizations are also preparing; Demand Justice is planning a multimillion-dollar effort to oppose potential Trump Supreme Court appointees before they happen.

The March 20 gathering at the National Constitution Center and daylong conference on Justice Alito’s tenure were sponsored by the Federalist Society, the conservative legal organization that played a leading role in recommending judicial nominees to Mr. Trump in his first term.

In attendance were many of those judges, now serving on appeals courts nationwide, which have served as a pipeline for Supreme Court nominees. Among them: Gregory Katsas and Neomi Rao of the D.C. Circuit; Steven Menashi and Michael Park of the 2nd Circuit; Stephanos Bibas of the 3rd Circuit; Andrew Oldham of the 5th Circuit and Amul Thapar of the 6th Circuit.

David Lat, who closely follows the hiring practices of the justices on his Substack, said Justice Alito seems “invigorated in some ways by the Trump administration, as its foremost defender,” along with Justice Thomas. He noted that justices have regularly served into their 80s and that Justice John Paul Stevens announced his retirement just days before his 90th birthday.

By Supreme Court standards, Mr. Lat said, Justice Alito is a “spring chicken.”