@glynmoody My current theory is that people left of center trend to do this because they see results. The GOP / right-wing know they're unpopular, don't care, they have a goal in mind and aren't going to be deterred by such things as disapproval or shame.
But, going after other Dems / progressives causes action and gets results. Never mind that the cumulative effect is to repeatedly shoot the overall "team" in the foot, by golly they feel like they've been heard.
I might be a touch cynical.