Fidel Castro observes the May Day parade at the Revolution Square in Havana, Cuba May 1, 1998.

Democratic front-runner Bernie Sanders came under fire Monday from his rivals over his recent partial defense of Cuban revolutionary and communist dictator Fidel Castro.

Less than two days after NBC News projected that Sanders won the Nevada caucus by a wide margin, his campaign foes tore into comments about Castro that he made during an interview with CBS' "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," tweeted former New York Mayor Mike Bloomberg.

"But sure, Bernie, let's talk about his literacy program."

Bloomberg TWEET

Bloomberg, whose net worth is estimated at over $60 billion, has already spent hundreds of millions of his own dollars on his presidential bid. He plans to unleash a full-on media onslaught against Sanders, including attack ads and the publication of opposition research, Bloomberg campaign aides told CNBC.

Bloomberg and other presidential hopefuls are ramping up their attacks against Sanders, who is poised to head into Super Tuesday on March 3 with more momentum and support in the polls than any other candidate

But some Democratic political watchers fear that Sanders, a 78-year-old democratic socialist, may be a risky choice to take on President Donald Trump in a general election.

Sanders told "60 Minutes" that while he opposed the "authoritarian nature" of Castro's regime, "it's unfair to simply say everything is bad."

"When Fidel Castro came into office, you know what he did? He had a massive literacy program," Sanders said in the interview, which aired Sunday night. "Is that a bad thing? Even though Fidel Castro did it?"

After a reporter noted that many dissidents had been jailed in Castro's Cuba, Sanders responded, "That's right, and we condemn that."

Sanders used the opportunity to contrast himself with Trump, who has been criticized for his at-times warm language toward foreign leaders with poor track records on human rights.

"Unlike Donald Trump, let's be clear ... I do not think that [North Korean leader Kim Jong Un] is a good friend. I don't trade love letters with a murdering dictator," Sanders said.

Russian President Vladimir Putin, added Sanders, is "not a good friend of mine."

Castro died in 2016.

Other Democratic candidates were quick to lash out.

"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," said former South Bend Mayor Pete Buttigieg in a tweet.

"We can't risk nominating someone who doesn't recognize this," Buttigieg said.

Buttigieg TWEET

Buttigieg, the youngest Democrat in the primary race, won more state delegates in the first-in-the-nation Iowa caucus and narrowly lost the popular vote to Sanders in the New Hampshire primary. But Buttigieg was projected to place third in Nevada, trailing former Vice President Joe Biden, who had once been seen as the Democratic frontrunner.

Biden, Bloomberg and Buttigieg all took shots at Sanders as the Nevada results started rolling in.

"Senator Sanders believes in an inflexible, ideological revolution that leaves out most Democrats, not to mention most Americans," Buttigieg said in Las Vegas on Saturday.

"If we choose a candidate who appeals to a small base – like Senator Sanders – it will be a fatal error," Bloomberg's campaign said in a statement.

Biden, meanwhile, took a swipe at Sanders less directly, by referencing The Washington Post's recent report that Russia is attempting to boost Sanders' campaign as part of its efforts to meddle in the 2020 election.

"We're going to have more help from Vladimir Putin, who wants somebody he doesn't think can beat Trump," Biden said at his own campaign event in Vegas on Saturday.

Sanders has denounced the Kremlin's reported attempts to meddle, once again, in a U.S. presidential contest.

Sanders also took heat from multiple Democratic lawmakers in Florida, home to the majority of the U.S. population of Cuban exiles who escaped Castro's authoritarian government.

"As the first South American immigrant member of Congress who proudly represents thousands of Cuban Americans, I find Senator Bernie Sanders' comments on Castro's Cuba absolutely unacceptable," Florida Democratic Rep. Debbie Mucarsel-Powell said on Twitter.

"The Castro regime murdered and jailed dissidents, and caused unspeakable harm to too many South Florida families," the House Democrat wrote.

Dem TWEET

Rep. Donna Shalala, also a Florida Democrat, tweeted that she hopes Sanders in the future "will take time to speak to some of my constituents before he decides to sing the praises of a murderous tyrant like Fidel Castro."

Shalala TWEET