The Tea Party, say what you will about them, gets a great deal of obeisance from Republicans for one simple reason: they will primary you if they don’t like how you’ve been voting, and they’ll probably win that primary. They are feared. Progressives are not feared, because they do not believe enough in their ostensible principles to act on them in an effective fashionI'd add "and they're willing to lose those primaries." Losing the Senate seats in Nevada and Delaware didn't weaken the movement conservatives.
This post has led to spirited commentary.