He is an unapologetic socialist and proud of it. Even his admirers admit that he lacks social skills, and he tends to speak in tirades. Yet that has not stopped him winning eight consecutive elections to the US House of Representatives.
"Twenty years ago when people here thought about socialism they were thinking about the Soviet Union, about Albania," Mr Sanders told the Guardian in a telephone interview from the campaign trail. "Now they think about Scandinavia. In Vermont people understand I'm talking about democratic socialism."
Democratic socialism, however, has hardly proved to be a vote-winning formula in a country where even the word "liberal" is generally treated as an insult. Until now the best showing in a Senate race by a socialist of any stripe was in 1930 by Emil Seidel, who won 6% of the vote.
John McLaughry, the head of a free-market Vermont thinktank, the Ethan Allen Institute, said Mr Sanders is a throwback to that era. "Bernie Sanders is an unreconstructed 1930s socialist and proud of it. He's a skilful demagogue who casts every issue in that framework, a master practitioner of class warfare."
When Mr Sanders, a penniless but eloquent import from New York, got himself elected mayor of Burlington in 1981, at the height of the cold war, it rang some alarm bells. "I had to persuade the air force base across the lake that Bernie's rise didn't mean there was a communist takeover of Burlington," recalled Garrison Nelson, a politics professor at the University of Vermont who has known him since the 1970s.
"He used to sleep on the couch of a friend of mine, walking about town with no work," Prof Nelson said. "Bernie really is a subject for political anthropology. He has no political party. He has never been called charming. He has no money, and none of the resources we normally associate with success. However, he learned how to speak to a significant part of the disaffected population of Vermont."
Mr Sanders turned out to be a success as mayor, rejuvenating the city government and rehabilitating Burlington's depressed waterfront on Lake Champlain while ensuring that it was not gentrified beyond the reach of ordinary local people. "He stood this town on its ear," said Peter Freyne, a local journalist.
"I tried to make the government work for working people, and not just for corporations, and on that basis I was elected to Congress," Mr Sanders said. He has served 16 years in the House of Representatives, a lonely voice since the Republican takeover in 1994. He has however struck some interesting cross-party deals, siding with libertarian Republicans to oppose a clause in the Patriot Act which allowed the FBI to find out what books Americans borrowed from libraries.
He says his consistent electoral success reflects the widespread discontent with rising inequality, deepening poverty and dwindling access to affordable healthcare in the US. "People realise there is a lot to be learned from the democratic socialist models in northern Europe," Mr Sanders said. "The untold story here is the degree to which the middle class is shrinking and the gap between rich and poor is widening. It is a disgrace that the US has the highest rate of childhood poverty of any industrialised country on earth. Iraq is important, but it's not the only issue."
In a state of just over 600,000 people he also has a significant advantage over his Republican opponent, Rich Tarrant, a businessman who has spent about $7m on his campaign. "Sanders is popular because even if you disagree with him you know where he stands," said Eric Davis, a political scientist at Vermont's Middlebury College. "He pays attention to his political base. He's independent and iconoclastic and Vermonters like that."