Mr. Sanders doubled down on his comments in a CNN town hall on Monday night, arguing that many people in Cuba were “illiterate” when Mr. Castro came to power.

“I think teaching people to read and write is a good thing,” he said, adding that he has been “critical of all authoritarian regimes all over the world,” including Cuba, Nicaragua, Saudi Arabia, China and Russia.

Sensing an opening to remind voters about Mr. Sanders’s more left-wing views, three of his opponents pounced on the Castro clip from “60 Minutes.”

“Fidel Castro left a dark legacy of forced labor camps, religious repression, widespread poverty, firing squads, and the murder of thousands of his own people,” Michael R. Bloomberg, the former New York mayor whose campaign ads have flooded the Florida television airwaves, wrote on Twitter. “But sure, Bernie, let’s talk about his literacy program.”

Pete Buttigieg, the former mayor of South Bend, Ind., tweeted: “After four years of looking on in horror as Trump cozied up to dictators, we need a president who will be extremely clear in standing against regimes that violate human rights abroad. We can’t risk nominating someone who doesn’t recognize this.”

Cristóbal Alex, a senior adviser to former Vice President Joseph R. Biden Jr., called Mr. Sanders’s comments “dangerous” and “deeply offensive to the many people in Florida, New Jersey and across the country that have fled political persecution and sought refuge in the United States.”

Mr. Sanders might have little to worry about in the Democratic race: Florida does not hold its primary until March 17, after bigger states such as California and Texas.