I don't exactly know how or when "Washington" became an insult in the American political vocabulary.

As an analogy for how ridiculous it is for people running for congress of accusing each other to be "Washington", I thought that maybe it would be like basketball players insulting each other for dribbling the ball. But it is actually possible to play a good basketball game without ever dribbling the ball. Its not possible to serve in congress without going to Washington.

I suppose that this is a difference between denotation and connotation. The denotation "Washington" fits everyone in congress. But apparently it has some connotation that is very insidious. But since it can't be quite pinned down what that connotation is, it becomes a very empty insult.