WASHINGTON — After 30 years of pivotal decisions on the U.S. Supreme Court, Justice Anthony Kennedy is about to make his biggest one yet.

For the second straight year, Kennedy, 81, is the focus of retirement speculation as the court approaches the late-June end of its term. A retirement by the court’s swing justice would give President Donald Trump his second Supreme Court vacancy to fill before Republicans’ Senate majority goes on the November election ballot. It also would drop a political bomb into what is already one of the country’s most divisive eras since the Civil War.

Although he has given no public indication he plans to depart, advocates on both sides are wondering whether Kennedy’s age and Republican roots could lead him to do so. It could trigger a rancorous confirmation battle and open the court to a sharp — and probably enduring — turn to the right.

A Trump-appointed successor could create the five-member majority that legal conservatives have envisioned for decades. The reconstituted court could overturn long-standing precedents, including the 1973 Roe v. Wade abortion-rights ruling. How far it goes would probably depend on Chief Justice John Roberts, who sometimes tempers his conservative leanings to try to protect the court’s institutional standing.

Whether Kennedy will retire is something known to, at most, a handful of people. Former law clerks say they can only speculate, questioning even whether the justice himself knows what he will do.

What faint signals Kennedy has sent point toward at least another year on the bench. He has hired his law clerks for the Supreme Court’s next term and, as usual, will spent July teaching a law school course in Salzburg, Austria. From the bench, he has been as engaged as ever.

“I thought he looked great during the arguments,” said lawyer Carter Phillips, who has argued more than 75 cases before the court, including an antitrust matter involving China on April 24. “So I don’t see anything specific as a health issue that would cause him to walk away.”

Nor has Kennedy shown any obvious signs of slowing down in his opinion-writing. Although he has issued only one majority opinion this term, a decision that curbed human-rights lawsuits, it was an unusually complicated ruling that required him to cobble together a majority out of a splintered court.

Kennedy’s most significant opinions are probably yet to come. He could cast the deciding vote in many of the court’s biggest remaining cases, including clashes over Trump’s travel ban, partisan gerrymandering and the speech rights of people who oppose gay marriage.

Kennedy has been the court’s pivot point since 2006. He sided with the court’s conservative wing to loosen campaign finance rules in the Citizens United case and to invalidate a core part of the Voting Rights Act. He joined with the liberals to legalize gay marriage and, for the most part, has backed abortion access.

“Justice Kennedy is the most influential justice in the history of the Supreme Court, and it’s probably not even close,” said Michael Klarman, who teaches constitutional history at Harvard Law School. “It’s almost hard to think of areas where the court divides 5-4 and Justice Kennedy is not the deciding vote.”

Kennedy himself succeeded a swing voter on the court in 1988, when the Senate confirmed him to take the seat of Justice Lewis Powell. The appointment came only after President Ronald Reagan’s controversial first nominee, Robert Bork, was rejected by the Senate and his second nominee, Douglas Ginsburg, stepped aside over reports of past marijuana use.

A Trump nomination to fill the Kennedy seat could lead to a confirmation fight as rancorous as the one over Bork. But unlike with Bork, Republicans control the Senate 51-49 and could confirm a nominee without any Democratic support. Democrats are trying to take control of the Senate in November, though they face a tough fight.

Potential Supreme Court nominees would include Washington-based federal appeals court judge Brett Kavanaugh, a former Kennedy clerk with close ties to the justice. Trump could also consider three federal judges he interviewed before selecting Neil Gorsuch to fill an earlier vacancy: William Pryor of Alabama, Thomas Hardiman of Pennsylvania and Amul Thapar of Kentucky.

Supreme Court justices typically reveal retirement plans as the court nears the end of its term. The most recent justice to step down, John Paul Stevens, made his announcement on April 9, 2010. The previous year, Justice David Souter revealed his plans publicly on May 1.