Jonathan Bernstein studies the current composition of the Senate:
Look at this graph, which I've linked to many times: the thin part of the distribution isn't at the ends; it's in the middle. That's especially true on the Republican side. There are four outliers to the right (Coburn, Bunning, DeMint, Inhofe), but after that, there's not much difference between the 5th most conservative Republican and, say, the 26th (Wicker).
Then, once you get close to the middle, the Republicans are strung out again, with Snowe. The same thing is basically true on the Democratic side; there are a handful of moderates, then a larger group of moderate liberals, and then the bulk of the caucus, who are mainstream liberals. There are no "15 'theological' senators from the Progressive Caucus." In fact, the 15th most liberal Senator (Schumer) appears to be statistically indistinguishable from the most liberal and also from the 33rd most liberal Senator. And, really, that sounds about right to me; with the possible exception of Sanders (whose voting record turns out to be to the right of Schumer, for what it's worth), I don't think there are a handful of Senators I'd think of as the "most" liberal.
The Dish 