DW Nominate has been a popular measure of “ideology,” perhaps too much so, as this article indicates.
There is ZERO evidence that the Democrats have or have not been becoming more moderate or less in terms of their “ideology.” While people bandy about DW-Nominate scores as if they show something about this “ideology” business, there is absolutely no insight about “ideology” that they provide. DW-Nominate is predicated on the premise that people in Congress vote primarily based on ideology, defined on a Euclidean map, and that their ideology map can be recovered by analyzing their voting patterns. In other words, DW Nominate only tells us something about how they voted, which we assume is the product mainly of their ideology.
So the polarization that we see in DW-Nominate scores simply tell us that most Democrats and most Republicans don’t vote on the same side. What exactly it is that makes them not vote together, we do not really know. We assert that they don’t because of “ideological” differences, we “know” of this ideological difference from the fact that they don’t vote together–thus a tautology.
I think this obsession with this “ideology” as an explanation is dangerously misleading. How this concept is measured is actively obscuring the fact that we don’t really understand what is behind polarization, by giving us a reason concocted out of simply the fact that there is a polarization as if it actually “explains” anything.