CHARLESTON, S.C. (AP) — Bernie Sanders is facing the greatest test of his presidential campaign as his Democratic rivals prepare to launch a series of attacks on the Vermont senator during what could be a pivotal debate on the eve of the South Carolina primary.

With mounting fear among the Democratic establishment that the self-described democratic socialist is on the verge of gaining a significant lead in the delegates needed to secure the nomination, several candidates are resorting to a last ditch effort to stop him. The day before Tuesday night’s debate in Charleston, they previewed their lines of attack in a series of digital or television advertisements.

Pete Buttigieg, the former mayor of South Bend, Indiana, highlighted Sanders’ call for a government-financed health care system as an example of his “polarization.” Former Vice President Joe Biden accused Sanders of trying to undermine President Barack Obama’s 2012 reelection. And former New York Mayor Mike Bloomberg assailed Sanders’ record on gun control.

The new wave of infighting came just hours before seven Democrats were set to meet for the party’s 10th — and perhaps most consequential — debate of the 2020 primary season. And it marked a seminal moment in Sanders’ political career. After spending decades as an outside agitator accustomed to attacking the party establishment, he’s suddenly the one on defense.

His handling of the pressure could be crucial in determining whether he stays at the top of the Democratic pack. During a town hall Monday night televised on CNN, Sanders said he expected the attacks. But he still seemed to be adjusting to his new status.

“It is a little funny to find myself as the so-called front-runner,” he said.

Other candidates also have a lot on the line for Tuesday’s forum. After a stumbling debate debut last week, Bloomberg is seeking an opportunity to regain his footing. Biden, meanwhile, is looking to make a big impression in a state where he was long viewed as the unquestioned front-runner because of his support from black voters.



WATCH: How 2020 Democrats are framing their campaigns after Sanders’ Nevada win

Campaigning in South Carolina the day before the debate, Biden predicted he would win “by plenty” on Saturday

Having finished on top in three consecutive primary contests — including a tie in Iowa — Sanders is eyeing a knockout blow, however. He has shifted new staff into the state from Nevada in recent days, expanded his South Carolina advertising and added events to his schedule.

Sanders senior adviser Jeff Weaver said there was an “air of desperation” to the fresh attacks on his candidate.

“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.”

One candidate who didn’t take Sanders on directly Monday: Sen. Elizabeth Warren. Though she shares many of Sanders’ liberal policies and could benefit if he were to stumble, she’s been reluctant to tangle with him throughout the campaign.

She was Bloomberg’s fiercest critic during last week’s debate and has signaled she may continue to target the former New York mayor Tuesday night.

Sanders may benefit most from the sheer number of candidates still in the race. There are still seven high-profile Democrats fighting among themselves — and splitting up the anti-Sanders vote — to emerge as the strongest alternative to him.

Heading into the debate, there was no sign that any of those candidates were close to getting out.

Minnesota Sen. Amy Klobuchar, who finished in a distant fifth or sixth place in Nevada over the weekend, announced plans to launch a $4.2 million ad buy across several Super Tuesday states.

Meanwhile, outside groups in both parties joined Sanders’ Democratic rivals in trying to take him down.

The Big Tent Project, a new pro-Democratic organization trying to derail Sanders’ candidacy, sought to undermine his support with African Americans.

“Socialist Bernie Sanders is promising a lot of free stuff,” the group wrote in a brochure sent to 200,000 black voters in South Carolina. “Nominating Bernie means we reelect Trump. We can’t afford Bernie Sanders.”

Republicans working to win back the House majority jumped on comments Sanders made in a CBS News “60 Minutes” segment aired Sunday in which he praised the late Cuban ruler Fidel Castro for establishing what Sanders called a “massive literacy program” when he took power.

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The House GOP campaign arm called on several House Democrats — including three facing reelection this fall in South Florida — to say if they’d support Sanders should he become the nominee. Members including Rep. Donna Shalala weren’t happy.

“I’m hoping that in the future, Senator Sanders will take time to speak to some of my constituents before he decides to sing the praises of a murderous tyrant like Fidel Castro,” the first-term Democrat tweeted.

Sanders forcefully stood by his comments during the CNN town hall, saying he’d criticized “authoritarian regimes all over the world,” including Cuba, Nicaragua and Saudi Arabia, But he added that, after Castro took power in 1959, “the first thing he did” was initiate a literacy program.

“I think teaching people to read and write is a good thing,” Sanders said. “That is a fact. End of discussion.”

Sanders was also in a dispute with the American Israel Public Affairs Committee, an establishment group that advocates for strong U.S.-Israel relations. Sanders said he would skip the group’s conference because he was concerned about the event giving airtime to “leaders who express bigotry and oppose basic Palestinian rights.”

Jonathan Greenblatt, CEO of the Anti-Defamation League, called that characterization “offensive” and “irresponsible.”

Trump was encouraging the chaos from afar. During a trip to India, the Republican president predicted a long and messy primary season ahead for his Democratic rivals.

“It could go to the convention, it really could,” Trump said. “They are going to take it away from Crazy Bernie, they are not going to let him win.”

He added, “I actually think he would be tougher than most of the other candidates because he is like me, but I have a much bigger base.”

Peoples reported from New York.