Bernie Sanders heads into Tuesday night's debate as the current frontrunner in hte Democratic primary, his rivals are stepping up their attacks against the Vermont senator. "We've trained our eyes on him," one Michael Bloomberg campaign aide told POLITICO.

On Monday, his opponents released a barrage of criticism in the form of tweets, attack ads, and statements after 60 Minutes showed old footage of Sanders appearing to praise Fidel Castro.

But Sanders wasn't the only target: a pro-Sanders group released an old video of Bloomberg presenting an award to Harvey Weinstein.

The attacks signify that the 2020 Democratic primary has entered a newer, nastier phase after over a year of candidates playing it (relatively) civil.

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Attack ads. Tweets linking a U.S. Senator with history's most infamous despots. Video of a presidential candidate with Harvey Weinstein. Leaked closed-door audio.

The knives are finally coming out in the Democratic primary ahead of Tuesday night's debate in South Carolina, with most of the jabs aimed squarely at Bernie Sanders, who is currently the man to beat after victories in New Hampshire and Nevada.

Monday's barrage of belligerent statements, ads, oppo dumps, and Twitter pile-ons signaled a newer, nastier chapter of the 2020 primary and likely served as a preview of Tuesday's debate.

The attacks on Sanders stepped up after 60 Minutes aired an interview that replayed footage from the 1980s where Sanders appeared to praise the late Cuban dictator Fidel Castro because, "he educated their kids, gave them health care, totally transformed the society, you know?"

Anderson Cooper pressed Sanders on those past comments. "We're very opposed to the authoritarian nature of Cuba but you know, it's unfair to simply say everything is bad," Sanders replied. "You know? When Fidel Castro came into office, you know what he did? He had a massive literacy program. Is that a bad thing? Even though Fidel Castro did it?"

Sander's opponents immediately pounced.

His comments on Castro were "part of a larger pattern throughout his life to embrace autocratic leaders and governments across the globe," a spokesman for former vice president Joe Biden, Cristobal Alex, said in a statement. The Biden campaign also seized upon a report in The Atlantic that Sanders had considered primarying Barack Obama in 2012, citing the article in an attack ad that ran on digital platforms in South Carolina, POLITICO reported.

"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," former South Bend mayor Pete Buttigieg tweeted, along with the clip of Sanders' 60 Minutes segment.

Nobody went farther than Michael Bloomberg, whose campaign tweeted out a much-criticized thread that imagined satirical conversations between Sanders and brutal dictators like North Korea's Kim Jong-Un and Syria's Bashar al-Assad.

"'Vladimir Putin is willing to poison anyone who disagrees with him, but have you seen how that guy looks without a shirt!! Mmm delish!' --- #BernieOnDespots", the Team Bloomberg account wrote. The tweet was singled out by critics as homophobic, and the Bloomberg campaign has since deleted the entire thread.

The former New York mayor wasn't safe, either. On Monday, a pro-Sanders group People for Bernie tweeted an old video of Bloomberg presenting an award to Harvey Weinstein, who was just found guilty on charges of third-degree rape and first-degree criminal sexual act. Bloomberg had awarded the producer and his brother, Bob Weinstein, with the Made in NY award in 2013, while he was still mayor of New York City.

Meanwhile, CNN published closed-door 2016 audio of Bloomberg calling the rise of progressives like Senator Elizabeth Warren "scary" and claiming that, if he were to run for president, his platform would be to "defend the banks." The audio had been provided by an anonymous source who claimed to have once worked for Goldman Sachs, CNN said.

Warren, who has knack for turning sharp criticism into triumphant slogans, created a new URL called "ScareMikeBloomberg.com" where supporters could donate to her campaign.

The sparring will likely move from social media to broadcast television, as the candidates take the stage on Tuesday ahead of South Carolina's February 29 primary, the outcome of which could make or break several of the candidates.

"The debate tomorrow night and the campaign in general … needs to be about one candidate and that's Bernie Sanders," Bloomberg campaign strategist Dan Kanninen told reporters on Monday, according to POLITICO.

Bloomberg's team previewed a strategy of attacking Sanders to POLITICO. As an unnamed top aide told the publication: "We've trained our eyes on him."