Charleston (United States) (AFP) – Democratic White House hopefuls rounded on leftist frontrunner Bernie Sanders at a feisty debate Tuesday, attacking him as too extreme for American 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…