WASHINGTON — For more than four decades, the demise of Cuban leader Fidel Castro would have been a cause for celebration among Cuban-Americans and a huge relief for American government leaders. Now, with Mr. Castro facing a serious medical situation, his decision to temporarily yield power isn’t likely to bring many significant changes even if it becomes permanent.

While he was once feared as the bulwark of a communist advance in the Western Hemisphere, Mr. Castro was pushed to the margins with the collapse of the Soviet Union in 1991 and the demise of communist governments elsewhere. He has been supplanted as the most vocal leftist leader in Latin America by Venezuela’s President Chavez.

“Castro was a man of the 20th century,” said a Colombian senator, Gustavo Petro, 46, a former guerrilla for the Cuba-backed Movimiento 19 de Abril, or M–19, which put down arms in 1991 to become a legal political party. “The 21st century demands new leaders and is posing new challenges for the upcoming political leaderships.”

The central feature of American policy toward Cuba — an economic and commercial embargo imposed in 1962 — isn’t likely to change immediately if Mr. Castro’s brother, Raul, takes permanent control of the regime, according to experts and administration officials. While Raul Castro is viewed as favoring some economic reforms, President Bush’s administration won’t budge unless there is political overhaul as well.

“For the dictator, Fidel Castro, to hand off power to his brother, who has been the prison keeper, is not a change,” the White House press secretary, Tony Snow, said on Tuesday. “There are no plans to reach out.”

Administration officials pointed to a July 10 report from Secretary of State Rice and Commerce Secretary Carlos Gutierrez promising millions of dollars in aid and economic assistance to Cuba “provided we are asked by a Cuban transition government that is committed to dismantling all instruments of state repression.”

Mr. Castro, who will be 80 on August 13, looms large in the history of American foreign policy and he is the last survivor among the world leaders who were in power during the early days of the long Cold War standoff between America and the Soviet Union.

Mr. Petro said Mr. Castro’s legacy, whether he continues in power after recovery or relinquishes control permanently, will be the example he set for other nations in the region.

“He taught Latin Americans to remain always independent and sovereign from the U.S,” Mr. Petro said.

The Latin American leader most closely hewing to Mr. Castro’s line today is Mr. Chavez, who has pushed his oil-rich country closer to American adversaries such as Iran and accused Mr. Bush of trying to topple his government. In Vietnam on Tuesday, Mr. Chavez called news of Mr. Castro’s condition “worrisome.”

“Castro is Chavez’s mentor and they have a very close personal relationship,” the director of the Americas program at the Center for Strategic and International Studies in Washington, Peter DeShazo, said. “If indeed, Fidel Castro were not on the scene, then that would be a personal blow for Chavez.”

Mr. DeShazo said it isn’t clear what would happen in Cuba if Mr. Castro doesn’t return as head of the government, because authority in the regime is so centralized in him.

“Clearly, if there’s a change in the key leader then the old regime to a certain extent is shaken,” he said.

“This is a real opportunity to create real change in our Cuban policy,” Senator Dodd, a Democrat of Connecticut and longtime critic of Mr. Bush’s anti-Castro policy, said. While it’s possible Mr. Castro’s brother may consolidate power in the short term, Mr. Dodd said, “Raul is not Fidel” and lacks the forceful charisma of the longtime Cuban leader.

News of Mr. Castro’s ill health brought celebrations among the Cuban exiles in Miami’s Little Havana section.

People of Cuban ancestry represent about 0.4% of the American population, according to the Census Bureau. Their influence belies the size of their community. Five Cuban-Americans serve in Congress and some of its business leaders are top fund-raisers for both Republicans and Democrats.

The locus of Cuban-American political power in America is Florida, the nation’s fourth most populous state and a key factor in presidential politics. Mr. Bush won election in 2000 after the U.S. Supreme Court halted a recount of disputed ballots in the state, leaving him with a 500-vote margin of victory there. That gave Mr. Bush 271 electoral votes, one more than needed to gain the presidency, even though he lost the national popular vote by almost half a million ballots.

Those who claim Cuban background comprise 5% of the state’s population. Three members of Florida’s House delegation and one of its senators — all Republicans — are of Cuban ancestry. Senator Menendez, a Democrat of New Jersey, also is Cuban-American.

“Last night, when the news flashed across the screen, I thought I was living in something that I had dreamed would happen but had never really seen occur,” Senator Martinez, who emigrated from Cuba at the age of 15 in 1962,said.

Mr. Martinez said that while he doesn’t believe Raul Castro, 75, will veer from his brother’s policies, there may be others in the government who are ready for change.

“If there is a flaw in all of our dealings it’s that we don’t have a lot of direct contact with people inside the island,” he said at the Capitol on Tuesday.

American business interests are eager for a chance to return to Cuba, a market America once dominated, the president of the National Foreign Trade Council, William Reinsch, said.

“There are a whole fleet of companies ready and waiting to go back,” Mr. Reinsch said. “It’s a real market and it used to be our market.”

A 2003 study by the U.S. International Trade Commission, an independent federal body, estimated America would have gained $652 million to $990 million a year in exports if American sanctions did not exist. That would represent 0.11% of total American exports, according to U.S. Census data.