While a generalization has the flaws of being, well, a generalization; I've noticed that this trope is at least more true than not when you qualify what TYPE of Democrats and Republicans you're talking about.

I think one is true of the representatives - Democrat constituents generally fall in line without question; whereas I think the other is true of the people - Republican voters generally fall in line without question.

The rot is deep for the constituents on either side, however. There's a LOT of incentive to preserve party/ideological status quo regardless of where you land.

well, what do the facts say? Thinking back to the last powerful Dem politician to get in trouble, Senator Bob Menendez of New Jersey, I don't recall any outpouring of support from him from his colleagues or diehard Dem voters.