Two comments on the question Yglesias raises about ideological purges.
First, it's just par for the course that liberals are upset that the Democratic Congress and Democratic president aren't liberal enough. Progressives think of 2000-2006 as dark times indeed, but conservatives at the time and looking back see only new federal controls in No Child Left Behind, a huge new entitlement commitment in Medicare expansion, and a spending explosion featuring massive numbers of earmarks. Conservatives reacted to those things, in part, by taking on moderate politicians (most famously in Rhode Island and Pennsylvania Senate contests). For their trouble, they wound up with very liberal Senator Whitehouse and votes-like-a-liberal Senator Specter. Just as conservatives managed to overlook their achievements under GOP Congresses, liberals are now overlooking the considerable achievements of the current Congress and President Obama and focusing on the compromises necessary to pass health care, or Obama's failures to date on things like DODT.
Second, while I think it would be completely nuts for liberals to go after Baucus or Landreau or Conrad, all of whom would likely be replaced by extreme conservatives, Joe Lieberman is obviously a special case, since he's far to the right of his state and barely a "real" Democrat at all in a formal sense. Still, as annoying as he is to liberals, pushing him to flip to the Republican conference would almost certainly wind up moving his voting pattern sharply to the right, and it's hard to see how that helps progressives. Connecticut and national Democrats should try to knock him out when they have a chance to do it at the ballot box, but until then I think the Senate leadership has been correct in playing along with him.