The case against Bernie Sanders, according to Barney Frank

It didn't take long for Bernie Sanders, who was new to Congress in 1991, to frustrate the very people with whom he might have collaborated.

"Bernie alienates his natural allies," then-Rep. Barney Frank (D-Mass.) told the Los Angeles Times just months after Sanders first took federal office. "His holier-than-thou attitude — saying in a very loud voice he is smarter than everyone else and purer than everyone else — really undercuts his effectiveness."

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.

[This New York Daily News interview was pretty close to a disaster for Bernie Sanders]

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.)