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HILTON HEAD ISLAND, S.C. -- Worried Democrats on Monday 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. And a new political group was spending big to undermine 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 organization trying to derail Sanders' candidacy. "Nominating Bernie means we reelect Trump. We can't afford Bernie Sanders."

The multi-pronged broadside just five days before South Carolina's first-in-the-South primary represents the Democrats' most aggressive attempt to knock Sanders down. It reflects growing concern within his party that the self-described democratic socialist is tightening his grip on the presidential nomination while they fear he's too extreme to defeat President Donald Trump this fall.

It also underscores the precarious state of 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, Sanders is also making a strong play here. If he can eat into Biden's base of support, that would raise fundamental questions about the future of Biden's candidacy.

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.

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."

Biden still predicted he would win "by plenty" in Saturday's contest, the first with a sizable black population to weigh in.

Beyond South Carolina, polls suggest Sanders will perform well when more than a dozen states vote in the March 3 Super Tuesday contests. That's when critics fear Sanders could build an insurmountable delegate lead.

Sanders was the focus of Buttigieg's first attack ad of the 2020 campaign. In the ad, which began running Monday as part of the former South Bend mayor's multimillion-dollar South Carolina advertising campaign, Buttigieg highlights Sanders' call for a government-financed health care system as an example of the Democratic front-runner's "polarization."

Biden released an online ad accusing Sanders of trying to undermine President Barack Obama's 2012 reelection campaign with a possible primary challenge. Sanders, of course, ultimately did not challenge 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 Bloomberg released a new ad of his own assailing Sanders' record on gun control, citing the senator's endorsement by the National Rifle Association when he first ran for Congress decades ago.

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

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

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.

Some of Warren's supporters say she has to start drawing a sharper contrast with Sanders if she's to break out after middling performances in the first three contests.

"She needs to get out tomorrow night and get after him," said Beth Maass, 79, referring to Tuesday's debate. "She's in the lane with Bernie and only one is going to make it out of the lane."

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.

There was no sign Monday 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.

Billionaire activist Tom Steyer has yet to spend money on an anti-Sanders campaign, but he went after him by name 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."

There were also new signs Monday that Sanders' emergence as the possible face of the Democratic Party in 2020 could cause problems for vulnerable House and Senate candidates across the country.

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.

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 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."

Rep. Elaine Luria, a Jewish Democrat from the Super Tuesday state of Virginia, issued a statement condemning Sanders' remarks, which she said "contribute to the divisive rhetoric often used when discussing the issue of Israel's right to exist."

Watching the tumult from afar during a trip to India, Trump predicted a long and messy primary season ahead for his 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."

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Peoples reported from New York and Barrow reported from Charleston, South Carolina. AP writers Alan Fram in Washington, Elana Schor and Julie Pace in Charleston, Jill Colvin with Trump and Thomas Beaumont in Des Moines, Iowa, contributed.