A State Department spokesman derided Castro's 76-year-old brother and heir apparent, Raul Castro, as a "Fidel lite" and "dictator lite."
Despite having wished openly for Castro's demise and the end of his rule for years, U.S. officials said Tuesday that the Cuban president's decision to step down on his own terms leaves little hope for real democratic transition in communist Cuba during President George W. Bush's final year in office, although it may open options for Bush's successor in the White House.
Castro said in his letter that he was not bidding his thousands of supporters farewell, and that he would continue to fight for the ideals he's espoused for almost 50 years, CBS News correspondent Kelly Cobiella reports.
Led by Bush, a chorus of officials expressed hope that Castro's departure would spark fundamental change for the Cuban people. They also said they doubted that would happen under Raul Castro and said there was little chance the nearly 50-year-old U.S. embargo on Cuba would be lifted.
"They're the ones who suffered under Fidel Castro," Bush told a news conference in Rwanda. "They're the ones who were put in prison because of their beliefs. They're the ones who have been denied their right to live in a free society. So I view this as a period of transition, and it should be the beginning of the democratic transition in Cuba."
"Eventually, this transition ought to lead to free and fair elections - and I mean free, and I mean fair, not these kind of staged elections that the Castro brothers try to foist off as true democracy," Bush said. "The United States will help the people of Cuba realize the blessings of liberty."
In Washington, senior State Department officials said there would be no lifting of the embargo, which has been the centerpiece of American policy toward Cuba since it was imposed in 1960 and strengthened in 1962.
"I can't imagine that happening any time soon," said Deputy Secretary of State John Negroponte.
The ailing Castro, 81, who has called the embargo "criminal" and claims its impact has run into the tens of billions of dollars, announced early Tuesday he would not accept another term in office when parliament meets to elect a new president this weekend.
Castro outlasted nine U.S. presidents who, with some minor policy adjustments, have steadily ramped up pressure on Cuba. At least two secretaries of state, Madeleine Albright and Colin Powell, said publicly while in office that they hoped "the actuarial tables" would catch up with the aging Cuban leader who was a persistent thorn in Washington's side.
Long-standing U.S. irritation with Castro was evident on Tuesday with officials stressing they were not optimistic for any kind of quick change under Raul Castro, to whom Fidel ceded power temporarily in July 2006.
"The changing of the guard is not significant of and by itself," said deputy State Department spokesman Tom Casey. "It will be significant if in fact it leads to greater openness and freedom for the Cuban people and ultimately to a democratic transition."
Casey said "the general analysis is that Raul Castro is 'Fidel lite.' He is simply a continuation of the Castro regime, of the dictatorship."
Jumping into the fray, the top three presidential candidates all said Washington should look for ways to encourage democratic change in Cuba, steps that could lead to normalizing U.S. relations with Cuba later on.
Democrats Hillary Rodham Clinton and Barack Obama and Republican John McCain also demanded the release of Cuban political prisoners.
"The United States must pursue an active policy that does everything possible to advance the cause of freedom, democracy and opportunity in Cuba," Clinton said.
Obama, who is waging a hard-fought campaign with Clinton for the Democratic nomination, said the United States must be prepared for normal relations with Cuba and an easing of the embargo if Cuba's new leader "begins opening Cuba to meaningful democratic change."
Castro's resignation "should mark the end of a dark era in Cuba's history. ... Fidel Castro's stepping down is an essential first step, but it is sadly insufficient in bringing freedom to Cuba," he said.
In a statement, McCain underscored that "freedom for the Cuban people is not yet at hand. We must press the Cuban regime to release all political prisoners unconditionally, to legalize all political parties, labor unions and free media, and to schedule internationally monitored elections,"
Republican Rep. Ileana Ros-Lehtinen, who was born in Havana, said Castro's resignation was irrelevant because his regime had already "done great harm to the suffering Cuban people."
Sen. Robert Menendez, a Democrat who also is of Cuban descent, said Castro's resignation "is not the cause for celebration that some would believe. This does not represent the replacement of totalitarianism with democracy. Instead, it is the replacement of one dictator with another."
Cuban leaders have often expressed willingness to deal with the United States, but only on Cuban terms. Those conditions look nothing like U.S. demands laid down to lift the embargo in the Helms-Burton Act of 1996.
The law grants U.S. presidents broad leeway over how to enforce the embargo. The embargo rules themselves stand as long as either Raul or Fidel are in power, according to Helms-Burton.
Still, Raul Castro has offered repeatedly to improve relations with Washington, even if the Bush administration shows no sign of taking him up on it. He has hinted that he favors greater, if still limited, economic freedom. And he already has allowed more, if limited, public criticism of the government.
Since taking the provisional presidency, he has extradited three U.S. fugitives, reduced the number of Cuban political prisoners by more than 20 percent and refrained from imposing the death penalty in two military mutinies where firing squads seemed likely. He also said Cuban forces would recapture any terror suspects who escape from the Guantanamo prison.
More uncertain is who will be chosen as Raul's new successor, although 56-year-old council Vice President Carlos Lage, who is Cuba's de facto prime minister, is a strong possibility.
"Raul is also old," allowed Isabel, a 61-year-old Havana street sweeper. She speculated that he could be succeeded by Lage, Foreign Minister Felipe Perez Roque, "or another younger person with new eyes."