Albert D. Cover, Neil Pinney and George Serra, "Voting Behavior in the US House and Senate: Regional Shifts and Contemporary Changes in Party Coalitions," Party Politics, 3 (April 1997), 221-241.

First Paragraph:
Although robust, congressional parties are part of the American political experience, political parties in the USA have been criticized for not being 'responsible'. Indeed, nobody is likely to confuse either of the major American parties with one of their more ideologically coherent European counterparts. The problem of establishing effective governing parties in the USA has troubled political scientists recently as it has in the past. Polsby (1983,1986) makes the point sharply when he discusses the impact of the plebiscitary nominating process on the ability of presidents, once nominated and elected, to work cooperatively with Congress. Moreover, many studies of the US Congress explain why the institution is not (and perhaps cannot be) dominated by stronger parties. These studies often emphasize the remarkable degree to which parties persist, notwithstanding limits imposed by the American system (e.g. Mayhew, 1974; Fiorina, 1989; Jacobson, 1992).

Figures and Tables:
Figure 1: House Democratic mean adjusted ADA scores by region.
Figure 2: Senate Democratic mean adjusted ADA scores by region.
Figure 3: House Republican mean adjusted ADA scores by region.
Figure 4: Senate Republican mean adjusted ADA scores by region.
Figure 5: House and Senate Democratic adjusted ADA dispersion scores.
Figure 6: House and Senate Republican adjusted ADA dispersion scores.
Figure 7: Regional differences in Democratic dispersion scores (Standard deviation of non-southern Democrats minus standard deviation of southern Democrats).
Figure 8: Regional differences in Republican dispersion scores (Standard deviation of non-eastern Republicans minus standard deviation of eastern Republicans).
Table 1: Cohesion scores.

Last Paragraph:
It remains to be seen, of course, how the party groups evolve. As long as these outlier groups exist, however, and as long as they remain distinctive from their mainstream counterparts, they remain a brake on movements toward cohesive, responsible parties in the USA. This resistance to overall party cohesion is, of course, exacerbated to the extent that the outliers are ideologically atypical within their respective parties. from this perspective their reduction to historical curiosities would not be mourned by those advocating a crisper delineation between American political parties.