Bernie Sanders, who is leading the Democratic pack in the contest to take on U.S. President Donald Trump in November, is the subject of fresh scrutiny and mounting attacks days ahead of the South Carolina primary.

The self-proclaimed "democratic socialist" Vermont senator is now the clear favorite to win the most delegates in the Democratic race, but moderates fear the 78-year-old's views and policy positions are too extreme to win a national election – he is likely to be the main target when the candidates gather on the debate stage in South Carolina on Tuesday evening.

The Vermont senator effectively tied for first place with Pete Buttigieg in the Iowa caucuses, but narrowly beat the former mayor in New Hampshire before winning a decisive victory in Nevada.

The latest polling also places Sanders well ahead of the Democratic field, with a CBS/YouGov poll suggesting he has support from 28 percent of those likely to vote in the Democratic primary.

The survey, conducted after last week's debate, puts Elizabeth Warren on 19 percent and Joe Biden on 17 percent, followed by Michael Bloomberg on 13 percent, Buttigieg on 10 percent and Amy Klobuchar on five percent.

Moderate candidates – who are splitting the anti-Sanders vote – are now intensifying attacks against the contest leader, with criticisms focusing on Sanders' stance on gun safety, the affordability of his plans, his approach to foreign policy, his electability and his relationship with former President Barack Obama.

Biden, the former vice president who faced a make-or-break contest in South Carolina on Saturday, has released a new digital ad accusing Sanders of considering a primary challenge to Obama ahead of the 2012 election. "When it comes to building on President Obama's legacy, Bernie Sanders just can't be trusted," the one-minute ad argues.

The Sanders campaign insisted the candidate never planned to stand against Obama in 2012.

Biden has long viewed South Carolina as the firewall which could reinvigorate his spluttering campaign because of his strong support among black voters – around 60 percent of the Democratic electorate in the state is black – but recent polling suggests Sanders is chipping away at that support.

Buttigieg launched his first attack ad of the campaign on Monday, accusing Sanders of "polarization" as part of a multi-million-U.S.-dollar campaign in South Carolina. 

Bloomberg, the billionaire former New York City mayor, has concentrated his attacks on Sanders on electability and gun safety. He is running social media ads criticizing the senator's past opposition to legislation that created new background checks on gun purchases and insinuating he has links to the National Rifle Association (NRA). 

"The NRA paved the road to Washington for Bernie Sanders," Bloomberg tweeted. "We deserve a president who is not beholden to the gun lobby." The Sanders campaign insisted the candidate had never accepted money from the NRA, and noted his "F" rating with the pro-gun lobby group.

Sanders has also come under fire from Republicans and fellow Democrats, particularly in the key state of Florida, home to a large Cuban-American population, for a partial defense of former Cuban leader Fidel Castro in a "60 Minutes" interview that aired on Sunday.

"When Fidel Castro came into office, you know what he did? He had a massive literacy program. Is that a bad thing? Even though Fidel Castro did it?" Sanders asked in the interview.

".@SenSanders comments on Fidel Castro are ill-informed & insulting to thousands of Floridians," tweeted Florida Representative Stephanie Murphy on Monday, in response to the comments.