The problem with Rosenthal and Poole's analysis is that they do not attempt to define what it *means* to be on one side of a liberal/conservative axis. You can certainly code votes on various bills as "left" or "right" on an imaginary x-axis, but I am not aware of any work that has been done showing that a politician's position on this axis can accurately predict their future votes, let alone that the axis has any substantive ideological content. Thus, the fact that congressional votes tend to clump in vague patterns that roughly match up with fuzzy ideas about what it means to be a liberal/conservative does not really "explain" variation in Congressional behavior any more than calling all people with odd habits "psychotic" and all people with ordinary habits "sane" explains variation in human behavior--no causal mechanism has been identified, and our expectations about the future have not been meaningfully constrained. I believe there are other studies that have confined themselves to variables that are easier to rigorously define, such as "attitude toward taxes," and have shown less clumping.