Democratic White House hopefuls rounded on leftist frontrunner Bernie Sanders at a feisty debate on Tuesday, attacking him as too extreme for US voters and a flawed challenger to President Donald Trump.

Joe Biden, who needs a victory in South Carolina’s crucial primary on Saturday to keep his campaign alive, hit Sanders as soft on gun control, while billionaire tycoon Michael Bloomberg claimed Russia was working to help Sanders win the nomination – betting he would be defeated in November.

And Sanders’ rivals joined in savaging the self-described democratic socialist as too radical to appeal to a broad swathe of US citizens.

Fellow progressive Elizabeth Warren and centrists Pete Buttigieg and Amy Klobuchar – all desperate to halt Sanders’ momentum before it is too late – laid into his ability to deliver on costly programmes such as universal health care and tuition-free college.

Buttigieg, a 38-year-old military veteran presenting himself as a unifier, warned a Sanders fight against Trump would spell “chaos” and divide the nation.

“I tell you what it adds up to. It ends up as four more years of Donald Trump,” Buttigieg said.

Sanders is in pole position in South Carolina, the last step before “Super Tuesday” on March 3 when 14 states vote and a whopping third of all delegates – the representatives who pick the nominee at the Democratic Party’s July convention – are up for grabs.

The 78-year-old hit back at the charge that his policies were too “radical”, insisting such ideas “exist in countries all over the world”, including the notion that health care is a human right.

“The way we beat Trump, which is what everybody up here wants, is we need a campaign of energy and excitement,” Sanders said.

“We need to bring working people back into the Democratic Party.”