"A lot of people here talk about what they believe in, but they don't act on it," Sen. Mark Warner said. "He always acts on what he believes. "¦ We can agree or disagree, but you know where he stands."

Lawmakers on both sides of the aisle, including Sanders himself, point to last year's deal to improve the disastrous, scandal-ridden Veterans Affairs Department as a highlight. After weeks of negotiating with a cadre of Republican colleagues, Sanders helped pass the deal on a 91-3 vote in the Senate. "In a pretty dysfunctional Congress I helped pass, in a bipartisan way, the significant veterans bill, which increases health care to veterans and lowers waiting times, and I'm proud of that," Sanders said. "That was a significant step forward."

"He knew when to hold and knew when to fold and, I think, maximized what we could get for veterans," said Sen. Chuck Schumer, who also participated in the VA talks.

Sanders has also passed an amendment to the Dodd-Frank bill that led to the first audit of the Federal Reserve. He and Sen. Robert Menendez secured funding in the 2008 stimulus bill for clean-energy initiatives. And he inserted language into the Affordable Care Act to increase funding for community health centers.

Those legislative wins are roughly on par with those of his fellow classmates of 2006, including more moderate members whose ideological leanings more easily lend them to compromise with the other side, such as Sens. Jon Tester and Bob Corker.

But as with Coburn, Sanders' willingness to stand up and say no has also helped him to score victories on Capitol Hill. Sanders highlights his battles to prevent Republicans from cutting Social Security benefits as well as "the complete decimation of the U.S. Postal Service."

Despite those rough moments, Republicans still say by and large they like the senator. Sen. Jeff Sessions, who served on the Budget Committee with Sanders, said that while the two couldn't be more opposite ideologically, they still share a mutual respect. "So often he would articulate the liberal — very liberal — line. And I would articulate the conservative line. And it would go something like, 'We need to tax the rich, we've got too many poor.' And I said, 'That's right. We've got so much government, so much taxes, we really, you know, created the poor. It's your problem,' " Sessions grinned. "But you know, I've always respected Bernie and we've gotten along personally well."

Sen. John McCain, who negotiated the VA deal with Sanders after Sen. Richard Burr, then the ranking member on the Veterans Committee, said he couldn't get any further in the negotiations with Sanders, gave the independent high praise, noting that "his word is good."

But he acknowledged that Sanders can be cantankerous, adding with a laugh: "Both of us have that reputation."