“When he served here he was like a lone ranger — I don’t recall who he worked with, or on what,” said Representative Anna Eshoo, a California Democrat who is one of House Speaker Nancy Pelosi’s closest allies. “I don’t know what he produced in the House — thin, very thin.”

He has had more influence in the Senate, up to a point.

“Some senators, like Hubert Humphrey and Teddy Kennedy, were good on both inside and outside strategy,” added Senator Sherrod Brown, Democrat of Ohio, a labor-allied progressive who briefly entertained running for president this year. “Bernie has chosen to pursue the outside strategy. The inside is a means to that end. Bernie clearly looks to build support from the outside. He works with his colleagues, but the outside, that is his strength.”

His work as ranking member of the Senate Budget Committee has been mixed. Mr. Sanders has had trouble recruiting and retaining first-class staff for the committee and has often called on Senator Patty Murray of Washington, a highly regarded legislative technician who served as committee chair, to lend the expertise of her aides on complex budget matters, according to three Democratic senators and two aides who spoke on the condition of anonymity.

Mr. Sanders bridles at boring technocratic toil, and once impatiently asked an aide, “What’s the news for today?” during a discussion of ho-hum committee business, one former staff member recalled.

Mr. Sanders said his main goal had always been to move the Democratic Party to the left, rather than work within the system to hammer out compromise measures. And he has had an impact on select issues, pushing the leadership of both parties to challenge President Trump on his support of Saudi intervention in Yemen. When asked for his most significant legislative accomplishment in the last year, he pointed to bills that would pressure large employers like Amazon, Disney and Walmart to improve their pay and benefits.

That measure is unlikely to see the floor, he conceded, but the “point of the legislation was to tell Amazon, to tell Disney, we are going to stand with workers — you can have significant success without ever passing a piece of legislation.”

Despite wearing his curmudgeon’s attitude with pride, Mr. Sanders has gone out of his way to maintain a positive working relationship with Ms. Warren, huddling with her in a Senate hallway recently to smooth over a fracas between the campaigns on Twitter.