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 tonight’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 today’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.

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.”