This applies for one of his Congressional runs:
An Old School Republican who became disgusted with the GOP's stance on social issues sometime during the Reagan era and didn't see Sander's economics as being as big of a problem.
Likely a single issue voter on some issue Congressional Sanders differed with their GOP opponent, who has kept this view for at least 60 years. My guess, Trade, New England was pretty Protectionist in the early 20th century, so I assume this voter was a single issue anti-Free Trade Voter in Vermont.