Mostly, he works.

“He is always working and looking to move the ball forward, even when he is having a casual conversation,” said Michaeleen Crowell, the former chief of staff in his Senate office. “It’s hard for him to take time off for himself. He’s happiest when he’s working.”

A ‘Lone Ranger’ Finds Allies

In January, Hillary Clinton, who believes Mr. Sanders’s refusal to quickly exit the 2016 primaries damaged her cause, made news with her comment that “nobody likes” Mr. Sanders in the Senate — ushering in a debate over whether he was simply too abrasive to get elected.

The claim was not entirely baseless. Mr. Sanders, an independent who caucuses with Democrats, was deeply unpopular for many of his 16 years in the House of Representatives, where he bucked his own leadership and pelted bills with amendments, many of the gadfly variety.

“He was like a lone ranger,” Representative Anna Eshoo, a California Democrat who is one of House Speaker Nancy Pelosi’s closest allies, said in an interview last summer. She called his record “thin.”

But the Senate has a higher threshold for grandstanding and eccentricity, so Mr. Sanders, who forged an alliance with the former majority leader Harry Reid, found a more forgiving crowd. He is less disliked than seen as a loner prone to delivering irritating sermons, fellow senators said.

“No, people don’t hate Bernie,” said a Democratic senator who spoke on the condition of anonymity. “I’d say the feeling is closer to resentment, because some of us have to take the tough votes on things so he can keep his brand pure.”

As a boss, Mr. Sanders is notoriously tough. Former aides have described him as demanding and particular, sometimes to a fault. In many ways, he is the quintessential Brooklyn diner patron: The service is never quite fast enough, and what they deliver is never quite what he ordered.