Garrison Nelson, who has been a political-science professor at the University of Vermont since 1968, says Mr. Sanders always oriented toward the interests of individual workers. “He’s an evangelical leftist,” Professor Nelson says.

From the beginning, that set him apart from the old-school Democrats who dominated Vermont’s small cities — Barre, Winooski, Rutland — where Italian, French-Canadian, Irish and Polish immigrants had come to work in the mills and quarries. It also set him apart from many of the new-style liberals who arrived in the state in the 1960s and 1970s, who concerned themselves mainly with the environment, energy, women’s rights and education.

Mr. Sanders fell into no camp but his own. By running for mayor, he challenged the old boys’ network that supported the incumbent Democrat, Gordon Paquette, alienating the party for years.

Later, when he decided to run for Congress, he knew he had to broaden his appeal. Vincent Illuzzi, a Republican state senator for 32 years, represented two counties in the Northeast Kingdom, the state’s poorest, remotest region, where voters are more conservative. He says that over the years Mr. Sanders has spent more time there than any other member of Vermont’s congressional delegation.

When Mr. Sanders showed up, he talked about how the banks or the cable companies were ripping people off, how the government was letting veterans down. He was there to help workers in their bid to unionize the local furniture mill.

He was not there to push same-sex marriage, and he was not going to emphasize those environmental issues that might cost a logger his job. And he definitely was not going to argue for gun control among voters who depended on their rifles to put meat on the table.

If Mr. Sanders had come across as a liberal, he would not have done as well as he did among voters in the Northeast Kingdom. But, as Professor Nelson says, he was not a liberal; he was a “populist prophet” who was able to capture the sense of unfairness that working people felt in their bones.