Worried Democrats have intensified their assault against the party’s presidential front-runner Bernie Sanders as the Vermont senator marched toward South Carolina’s weekend primary eyeing a knockout blow.

At least three leading candidates, Joe Biden, Pete Buttigieg and Mike Bloomberg reinforced their anti-Sanders rhetoric with paid attack ads for the first time.

US Election graphic
(PA Graphics)

And a new political group was spending big to undermine Mr Sanders’ standing with African American voters.

“Socialist Bernie Sanders is promising a lot of free stuff,” says a brochure sent to 200,000 black voters in South Carolina by The Big Tent Project, a new organisation trying to derail Mr Sanders’ candidacy.

“Nominating Bernie means we reelect Trump. We can’t afford Bernie Sanders.”

Joe Biden
Joe Biden speaks at a campaign event in Charleston, South Carolina on Monday. The former vice-president’s nomination campaign has taken a battering as Bernie Sanders continues to surge (Patrick Semansky/AP)

While uncoordinated, the multi-pronged broadside just five days before the first Southern primary represents the Democrats’ most aggressive attempt to knock Mr Sanders down.

It reflects growing concern within his party that the self-described democratic socialist is tightening his grip on the presidential nomination amid fears from some that he is too extreme to defeat President Donald Trump this autumn.

It also underscores the precarious state of Joe Biden’s campaign.

The former vice president has long been viewed as the unquestioned front-runner in South Carolina because of his support from black voters.

But as the contest nears, Mr Sanders is also making a strong play in the state. If he can eat into Biden’s base of support, that would raise fundamental questions about the future of Biden’s candidacy.

Mr Sanders has shifted new staff into the state from Nevada in the last 24 hours, expanded his South Carolina advertising and added events to his schedule.

“There’s an air of desperation about it,” Mr Sanders senior adviser Jeff Weaver said of the fresh attacks.

“You’ve got candidates, you’ve got super PACs (political action committees), all piling on to stop Bernie Sanders. They know he has the momentum in the race.”

Mr Biden still predicted he would win “by plenty” in Saturday’s contest, the first with a sizeable black population to weigh in.

Regardless of whether Mr Sanders scores an upset in South Carolina, polls suggest he will perform well when more than a dozen states vote in the March 3 Super Tuesday contests. That’s when critics fear Mr Sanders could build an insurmountable delegate lead.

Election 2020 Pete Buttigieg
Pete Buttigieg, who has launched attack advertising against Bernie Sanders, speaks in North Charleston on Monday (Matt Rourke/AP)

Reflecting his growing focus on Sanders in recent days, Mr Buttigieg released his first attack ad of the 2020 campaign, highlighting Mr Sanders’ call for a government-financed health care system as a example of the Democratic front-runner’s “polarisation”.

At the same time, Mr Biden released an online ad accusing Mr Sanders of trying to undermine President Barack Obama’s 2012 reelection campaign with a possible primary challenge. Mr Sanders ultimately did not challenge Mr Obama from the left.

“When it comes to building on President Obama’s legacy, Bernie Sanders just can’t be trusted,” the Biden ad says.

And Mr Bloomberg released a new ad of his own assailing Mr Sanders’ record on gun control, citing Mr Sanders’ endorsement by the National Rifle Association when he first ran for Congress decades ago.

While he once had the NRA’s backing, Mr Sanders proudly proclaims his “F” rating from the pro-gun organisation now. And just last week, several gun control advocates who survived the Parkland, Florida, school shooting endorsed him.

Still, Mr Bloomberg tweeted: “The NRA paved the road to Washington for Bernie Sanders. We deserve a president who is not beholden to the gun lobby.”

They’ve begun running attack ads again him, but Mr Sanders may benefit most by 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 – in trying to emerge as the strongest alternative to him.

There was no sign on Monday that any of the eight candidates still in the race was close to withdrawing.

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

Billionaire activist Tom Steyer has yet to spend money on an anti-Sanders campaign, but he went after him by name on Monday before more than 100 voters at a breakfast in Hilton Head.

Steyer warned, “We can’t nominate someone who is going to divide us.”