Frank tempered that assessment just a few months later: "Collegiality didn't come easily," he told the Times. "But he now fits in. He's very much an outsider, but not an outsider in the sense that he is isolated."
Yet, a quarter-century later, Frank's criticism of Sanders endures.
Why has it been so hard for a fellow liberal from New England to get behind Sanders? It's not his principles, Frank said, but rather his approach.
“Is pragmatism the opposite of idealism? Or is pragmatism a necessary adjunct to idealism?” Frank, a strong Hillary Clinton supporter, said in a Tuesday interview with The Washington Post.
"I think Bernie Sanders tends to have the approach, 'Don't be pragmatic, state your ideals, state what you think is the right policy and be very wary of compromise and of accepting less than you want,'" he said, echoing comments he made on MSNBC the night before. "My view has been to fight hard for the leftward, most achievable results."
To Frank, who retired from Congress in early 2013, Sanders's methods are not just fruitless; they distort expectations of a system designed for compromise.
"Bernie Sanders has been in Congress for 25 years with little to show for it in terms of his accomplishments, and that’s because of the role he stakes out," Frank told Slate in a piece published last week. "It is harder to get things done in the American political system than a lot of people realize, and what happens is they blame the people in office for the system."
To the contrary, Sanders has defended his record, arguing that he played an especially active role in legislating through amendment. Indeed, fact-checking service Politifact found that he passed 17 amendments by recorded roll call votes from 1995 to 2007 — more than any other House member during that time. (He graduated to the Senate in 2007.)
The record aside, Frank's grievances with Sanders run deep.
"He has now become critical of those who compromise, he's critical of the pragmatic approach to getting things done and even suggests — this is the worst of it with regards to Hillary, but it applies to the rest of us — that we do it for base motives," Frank said in the Tuesday interview.
It's an argument Frank has made before — and one that he has suggested he would make even if he didn't support Clinton.
Sanders paints with too broad a brush, Frank wrote in a Politico column in February, when he criticizes the cozy relationship between Wall Street and Congress without defending Democrats strongly enough.
"It’s not that liberals object because Sanders advocated — or advocates now — going further," he said. "Most of us agree and are trying to do so. Our disagreement with him is on how to do it, and on what we believe to be the negative consequences of his approach."
Frank has held that view for years, long before he started cheerleading for Clinton's 2016 campaign.
"Frank says he came to like and work well with Sanders, with whom he served on the House Financial Services Committee," New York Times correspondent Mark Leibovich wrote in a Sanders profile in 2007:
His early objections were over Sanders’s railing against both parties as if they were the same. “I think when he first got here, Bernie underestimated the degree that Republicans had moved to the right,” Frank told me. “I get sick of people saying ‘a curse on both your houses.’ When you point out to them that you agree with them on most things, they’ll say, ‘Yeah, well, I hold my friends up to a higher standard.’ Well, O.K., but remember that we’re your friends.”
Among the legislation Sanders has come to criticize: The Dodd-Frank Act, which he voted for in 2010 but now says didn't go far enough.
"Dodd-Frank did not end much of the casino-style gambling," Sanders said last year, of the bill originally introduced by Frank as "The Wall Street Reform and Consumer Protection Act of 2009."
For his part, Frank told Slate: "There have been a couple of cases of Republican senators trying to weaken the Dodd-Frank Act. Elizabeth Warren has been a much more successful defender of that bill than Sen. Sanders has been."
Still, Frank respects the role Sanders plays. He credits the senator for remaining an outsider so many years after joining Congress.
"Substantively, he has consistently, forcefully and cogently made the case for a larger federal government role in improving both the fairness and the quality of life in our country, refusing to soft-pedal in the face of declining support for this view in public opinion," Frank wrote for Politico last July.
But it is that identity that precludes him from being a viable presidential candidate, Frank then argued.
"His very unwillingness to be confined by existing voter attitudes, as part of a long-term strategy to change them, is both a very valuable contribution to the democratic dialogue and an obvious bar to winning support from the majority of these very voters in the near term," he said.
As a fellow liberal, he can stand by Sanders's ideology, but not his approach. "The problem is not his rejection, basically, of the pragmatic element in implementing your ideals, but his criticism of the people who do follow that," he said Tuesday.