Carol E. Lee, Jose de Cordoba, Felicia Schwartz, Wall Street Journal–

WASHINGTON—The death of Fidel Castro raises questions about the future of President Barack Obama’s effort to restore U.S. relations with Cuba, as both countries undergo momentous political transitions.

In the U.S., President-elect Donald Trump takes office in January after sending mixed messages during the campaign about how he would approach Mr. Obama’s policy of restoring diplomatic relations with Cuba—at times saying he would reverse the effort, much of which has unfolded through executive orders that could be reversed.

“Fidel Castro is dead!” Mr. Trump wrote on Twitter, suggesting a celebration of the Cuban leader’s passing.

He added in a statement: “Though the tragedies, deaths and pain caused by Fidel Castro cannot be erased, our administration will do all it can to ensure the Cuban people can finally begin their journey toward prosperity and liberty.”

In Cuba, Mr. Castro’s death marks the end of an era that brought U.S. relations to a hostile standstill. And although Mr. Castro’s brother, Raul, has led the country for much of the past decade, the absence of the towering figure of the Cuban revolution could still affect U.S. relations.

Mr. Obama, who in March became the first sitting U.S. president to visit Cuba in 88 years, said Americans “extend a hand of friendship to the Cuban people” following Mr. Castro’s death.

“We know that this moment fills Cubans—in Cuba and in the United States—with powerful emotions, recalling the countless ways in which Fidel Castro altered the course of individual lives, families, and of the Cuban nation,” Mr. Obama said in a carefully worded statement. “History will record and judge the enormous impact of this singular figure on the people and world around him.” ‎

Pedro Freyre, who heads the international practice at law firm Akerman LLP, and represents several U.S. clients seeking to do business in Cuba, believes Mr. Castro’s death could help push normalization.

“If there was one person who embodied the Cuban revolution, it was Fidel Castro, and now he’s gone,” Mr. Freyre said, adding that in Russia, China and Vietnam “it wasn’t until the revolutionary leader was gone that real change could be instituted.”

Yet divisions among U.S. political leaders were underscored almost immediately after Mr. Castro’s death was announced, raising the prospect that opponents of Mr. Obama’s policy could try to take advantage of this moment to halt advances in relations between the two countries.

Sen. Marco Rubio of Florida, a Cuban-American and Republican presidential candidate this year, said in a statement: “Sadly, Fidel Castro’s death does not mean freedom for the Cuban people or justice for the democratic activists, religious leaders, and political opponents he and his brother have jailed and persecuted. The dictator has died, but the dictatorship has not…The future of Cuba ultimately remains in the hands of the Cuban people, and now more than ever Congress and the new administration must stand with them against their brutal rulers and support their struggle for freedom and basic human rights.”

Sen. Bob Menendez (D., N.J.), a Cuban-American who has opposed Mr. Obama’s policy, said Mr. Castro’s death “represents an historic opportunity” for the United States.

“Contrary to the romanticized idea being peddled by some, recent lopsided concessions in U.S. policy towards Cuba have not led to an iota of positive changes in the way the regime rules or the Cuban people live,” Mr. Menendez said in a statement. “We know that the Castro regime is still a brutal totalitarian dictatorship that continues to deprive the Cuban people of the basic human rights we so proudly proclaim to support around the world.”

Mr. Obama has long seen the sunset on the Castro regime as an opportunity for charting a new path for relations.

He campaigned in 2008 on softening U.S. policies toward Cuba and, once he won a second term in 2012, he authorized a secret effort to re-establish relations. In December 2014, the U.S. and Cuba announced they had reached a deal to do so.

Yet the president’s effort has met resistance in Congress, leaving him to implement it through executive actions that could be undone by his successor. Lawmakers have refused to lift the U.S. economic embargo against Cuba, which the White House has said is the only way to fully restore ties between the two countries.

Critics of Mr. Obama’s policy have also argued that he has not pressed Cuba enough on human rights concerns and the Castro regime’s efforts to stifle political dissent.

The White House has been working to implement new measures designed make U.S.-Cuba relations as difficult as possible for Mr. Trump to undo.

Those initiatives focus on strengthening economic ties, which the White House sees as its best option, given the re-establishment of relations was done through executive action and business partnerships.

His policies have made it easier for Americans to travel to Cuba and for U.S. companies to do businesses there.

Mr. Obama did not visit with Fidel Castro during his trip to Havana. Instead he spent several days with Raul Castro, including hours of meetings, a state dinner and a baseball game between the Cuban national team and the Tampa Bay Rays. Since taking power in 2008, Raul Castro had sidelined many of his older brother’s stalwarts from power.

Mr. Trump’s contradictory statements about normalization over the past year don’t lend much insight into how he might respond to Mr. Castro’s death. In the later stages of his campaign he took a harder line.

One sign of Mr. Trump’s intentions towards Cuba policy may be the recent naming of Mauricio Claver-Carone, a conservative, anti-embargo lobbyist and director of the U.S.-Cuba Democracy Political Action Committee, to his transition team at the Treasury Department. The Treasury Department is in charge of enforcing embargo violations.

The appointment of Mr. Claver-Carone, who did not immediately answer an emailed request for comment, was widely seen by Cuban Americans as a signal that Mr. Trump is serious about reversing Cuba engagement.

The move also “alarmed the Cuban government,” said Richard Feinberg, a fellow at the Brookings Institution who recently wrote a book about Cuba’s changing economy.

One possible move: to rescind the permission for U.S. travelers to bring back as many cigars and as much rum as they wanted to—which would be more a symbolic gesture than anything else. The U.S. might also cut back on the recently inaugurated scheduled airline service to Cuba.

Yet Mr. Obama’s policy of engagement is popular with most parts of Mr. Trump’s Republican coalition, said Javier Corrales, a Cuba expert at Amherst College in Massachusetts.

“All the fundamental interests—shipping companies, the farm lobby, oil, construction, Wall Street, and evangelicals, support it,” Mr. Corrales said. “Trump is capable of anything, but if he tries to set the policy back, he will enter into a big conflict with important parts of his coalition.”

Some observers view Mr. Castro’s death at 90 years old as the removal of a main impediment to change in Cuba.

While Raul, known as a pragmatist, had pushed modest economic reforms, Mr. Castro since his official retirement from power in 2008, had become the embodiment of resistance, often sniping at the younger Castro’s reform efforts by writing columns, called Reflecciones, full of veiled—and some not-so-veiled—criticisms of the reform process.

One notable example of this was a column published by the elder Castro in Granma, the official Communist Party newspaper, days after Mr. Obama’s historic visit to Havana last year.

“My modest suggestion is to reflect and do not try now to develop theories about Cuban politics,” Mr. Castro admonished the president. Cuba “has no need of gifts” from the United States, Mr. Castro said.

Mr. Feinberg said Mr. Castro’s presence held back reform in Cuba by igniting the passions of Cuban-American hardliners seeking to keep the U.S. embargo in place. With his death, “one of history’s longest grudge matches has finally been ended by biology,” Mr. Feinberg said.

Mr. Freyre said that while Mr. Trump had received the backing of anti-embargo Cubans in Miami, their vote was not consequential in the election.

“It’s a balance between the perceived stake of conservative Cubans, and the U.S. business community which has a foot in the door in Cuba,” he said. “Fidel is gone. Raul will step down in a year. The push to normalize continues to be overwhelming.”

Indeed Mr. Obama’s Cuba policy was aimed at grooming a new generation of Cuban-American voters, who largely support renewed relations, to back Democrats after decades of Republicans’ banking on their support.

One pollster, Latino Decisions, said that Cubans had gone for Mr. Trump at a higher rate than all Hispanics, by a margin 52% to 47% for Mr. Trump. But the margin did not define the election in Florida, a state Mr. Trump won.

In his statement on Mr. Castro’s death, Mr. Obama urged Cubans to “recall the past and also look to the future.”

“As they do, the Cuban people must know that they have a friend and partner in the United States of America,” Mr. Obama said.

Write to Carol E. Lee at, Jose de Cordoba at and Felicia Schwartz at