CHARLESTON, S.C. — Bernie Sanders is poised to face perhaps the toughest debate of his political life Tuesday night in South Carolina, as his rivals for the Democratic presidential nomination signal they will attack his record on socialism, guns and much more in a bid to arrest his momentum before he can clear a path to the nomination.

At the debate a week ago, Sanders tangled with rivals but many of them were focused on taking down billionaire Michael Bloomberg, who joined them on stage for the first time. But with Sanders now the undisputed front-runner following his sweeping victory in Nevada, virtually everybody else in the field is desperate to prevent the ornery democratic socialist from carrying his winning streak through South Carolina this Saturday and into Super Tuesday -- while amassing a delegate lead that could be exceedingly difficult to close.

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In a blistering attack, an aide from one rival campaign told Fox News on Tuesday that Sanders “will have more to answer for than his love for murderous communist regimes or his broken promise to release his health records.”

The aide added, “He’s also the only candidate who wrote an essay joking about rape.”

This was a reference to an essay written by Sanders in 1972 in which he said that women fantasize about being raped. It's one of countless examples of controversial remarks and writings from Sanders' past that have largely been glossed over by rivals in the run-up to his claiming front-runner status.

That could change tonight.

Underscoring the political target on his back, Sanders will be center stage between six other candidates at the CBS-hosted debate in Charleston. Hours before the debate is set to start at 8 p.m. ET, Democratic candidates were signaling their intent to go after Sanders hard.

Bloomberg is preparing a massive media campaign against Sanders, which will include opposition research, surrogates, and advertisements across multiple platforms. The efforts began Monday, as the Bloomberg campaign unveiled a video accusing Sanders of being weak on gun control. Bloomberg has already spent over a half-billion dollars in campaign advertising, but until now much of that was directed toward attacking President Trump and touting his own record.

Ahead of last week’s debate, an internal Bloomberg campaign memo aired concerns that Sanders could become unstoppable.

While Sanders essentially tied for the win in Iowa and claimed victory in New Hampshire, his weekend win in the Nevada caucuses drove home the reality that his rivals will have diminishing chances to halt that momentum.

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Meanwhile, former Vice President Joe Biden, as part of a new $600,000 digital ad buy in South Carolina, this week launched a new ad, first reported by Politico, which slammed Sanders for reportedly attempting to launch a primary bid against former President Barack Obama in 2012.

“When it comes to building on Barack Obama’s legacy, Bernie Sanders just can’t be trusted,” the ad warned. “Bernie Sanders was seriously thinking about challenging our first African American president in a primary.”

The Sanders campaign, in a statement to Politico, denied that the Vermont senator ever had presidential aspirations in 2012.

Former South Bend, Ind., Mayor Pete Buttigieg also slammed Sanders this week, claiming that Sanders’ call for “Medicare-for-all” was an example of his “polarization.”

A frequent topic Tuesday night could be Sanders' comments about communism. The Vermont senator took heat this week for remarks on CBS News' "60 Minutes" defending the late Cuban dictator Fidel Castro's policies, saying "it's unfair to simply say everything is bad." Sanders touted social welfare programs introduced under Castro's regime that he described as redeeming, despite the communist dictator's often repressive human rights violations against Cubans.

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"We need a president who will be extremely clear in standing against regimes that violate human rights abroad," Buttigieg tweeted. "We can't risk nominating someone who doesn't recognize this."

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And Bloomberg tweeted: "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. But sure, Bernie, let’s talk about his literacy program."

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Minnesota Sen. Amy Klobuchar and wealthy progressive advocate Tom Steyer could also take swings at Sanders, though it's unclear how aggressive Sen. Elizabeth Warren of Massachusetts-- who last week blistered Bloomberg and other rivals but until recently had something of a truce going with Sanders -- will be.

But the Sanders campaign doesn’t seem concerned, as senior adviser Jeff Weaver cast the expected incoming as simply an “air of desperation.”

“You’ve got candidates, you’ve got super PACs, all piling on to stop Bernie Sanders,” Weaver said. “They know he has the momentum in the race.”

The prospect of Sanders leveraging that momentum to build a daunting delegate lead on Super Tuesday has also set off alarm bells in the party, with critics warning that his nomination would pave the way for President Trump’s re-election and potentially hurt Democrats' chances of maintaining the majority in the House or taking the majority in the Senate.

As for South Carolina's primary on Saturday, the latest RealClearPolitics polling average reflects a tightening race between Sanders and Biden, with Biden remaining in the lead with 26.8 percent of the vote. Sanders, though, is nipping at his heels with 21.7 percent.

Fox News’ Gregg Re and The Associated Press contributed to this report.