Senator Bernie Sanders is under attack from rival US Democrats as he cements his position as a frontrunner to take on Donald Trump in November's election.
Seven candidates are debating in South Carolina on Tuesday, just days before the state becomes the fourth to vote on the party's nominee.
Several have launched fresh attack ads on Mr Sanders, who they see as too left-wing to win against Mr Trump.
The current frontrunner identifies himself as a democratic socialist.
Ahead of the 10th debate of the 2020 election cycle, Pete Buttigieg, Mike Bloomberg and Joe Biden all released advertisements targeting Mr Sanders decades-long record in the US Senate.
Some of Mr Sanders's signature campaign policies - such as government-provided healthcare, universal childcare and free university education - have been condemned by critics as too expensive and likely to alienate US voters.
What are the candidates saying?
Mr Bloomberg, a former New York City mayor, claims in a 90 second-advert that the National Rifle Association (NRA) "paved the road to Washington" for Mr Sanders.
"He spent the next three decades making sure they got a return on their investment. We deserve a president who is not beholden to the gun lobby," the ad says, referring to the gun-rights group that is strongly disliked by many liberal Democrats.
The ad also spotlights the Vermont senator's mixed record on support for gun control, including his vote against a 1993 bill that made background checks required for most gun sales.
In response, a group supporting Mr Sanders - People for Bernie - released video clips of Mr Bloomberg praising disgraced Hollywood film producer Harvey Weinstein, who has been convicted of rape.
On Monday, former Indiana Mayor Pete Buttigieg attacked Mr Sanders plan for government-provided healthcare in an ad that declares "instead of polarisation, progress".
Former Vice-President Joe Biden accused Mr Sanders of attempting to thwart the 2012 re-election attempt by President Barack Obama.
The Big Tent Project, a newly formed Democratic group that seeks to derail Mr Sanders's candidacy, has been distributing flyers in South Carolina, before voters go to the polls on Saturday.
"Socialist Bernie Sanders is promising a lot of free stuff," says a brochure that was sent to 200,000 black voters across the state, according to the Associated Press News Agency.
"Nominating Bernie means we re-elect Trump. We can't afford Bernie Sanders."
Too little, too late, from too many directions
Bernie Sanders's Democratic rivals are starting to treat him as a serious threat to win the party's nomination.
They're hitting him from the left on gun control and from the centre on foreign policy and the electability of his big-government proposals.
The stepped-up rhetoric suggests Tuesday night's debate is going to be an acrimonious affair, reminiscent of last's week's dog-pile on former New York Mayor Michael Bloomberg.
The problem for those who hope to stop Sanders is that their attacks may be coming too little, too late and from too many directions.
While they could damage the Vermont senator, unless one opponent clearly benefits from a Sanders slip, no-one will benefit. And at the moment, the anti-Sanders field is still too fractured to suggest one candidate can break from the pack.
If there's a silver lining for Sanders to the onslaught headed his way, it's that this will be a good test of his ability to hold up to fire if he becomes the Democratic nominee. It's better to sharpen his campaign's defences now than in October, when Donald Trump and the Republicans will show no mercy.
On Monday, Mr Bloomberg's campaign accused Mr Sanders of not doing enough to halt acts of vandalism against his campaign offices across the country.
“Eat the Rich”— Team Bloomberg (@Mike2020) February 24, 2020
Our office in Flint, MI
America deserves better. pic.twitter.com/ar5jKPmIJs
After the word "oligarch" was spray-painted at a campaign office in Chicago on Sunday, a spokesman pointed fingers directly at Mr Sanders.
"While we do not know who is directly responsible, we do know Senator Bernie Sanders and his campaign have repeatedly invoked this language, and the word 'oligarch' specifically when discussing Mike Bloomberg," Bloomberg campaign manager Kevin Sheekey said in a statement.
"This needs to end before someone gets hurt."
Mr Bloomberg's campaign also hit Mr Sanders over a CBS interview in which he expressed general dislike for the Fidel Castro's communist regime in Cuba, but praised its historical commitment to literacy programmes.
"We're very opposed to the authoritarian nature of Cuba but you know, it's unfair to simply say everything is bad. You know?" he said in a 60 Minutes interview that aired on Sunday, leading to criticism from Cuban exile communities in the US.
"Fidel Castro left a dark legacy of forced labour camps, religious repression, widespread poverty, firing squads, and the murder of thousands of his own people," Mr Bloomberg wrote in a tweet.
"But sure, Bernie, let's talk about his literacy program."
Mr Trump also weighed in during his trip to India.
"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."
Why does South Carolina matter?
The state is the biggest thus far to vote before March, and the one with the largest percentage of African-American voters.
Mr Biden is counting on support from African-American voters - a demographic largely missing from the early contests in Iowa, New Hampshire and Nevada.
A poll by CBS/YouGov in the state last week found Mr Biden retains a narrow lead with 28%, with Mr Sanders at 23% and billionaire Tom Steyer at 18%. The same poll in November had put Mr Biden with 45% of the vote to Mr Sanders's 15%.
Mr Steyer's numbers in South Carolina suggest he may be drawing African-American voters away from Mr Biden - something Mr Biden himself has pointed out, blaming "the amount of money being spent by the billionaires".
On the heels of the South Carolina vote is Super Tuesday - 3 March - when 14 states, including California and Texas, vote. By the end of Super Tuesday, it may be much clearer who the Democratic candidate will be.