Part III of this book investigates institutional change in Congress, which is an intermediate step between constituency change (as analyzed in part II) and party polarization (as described in part 1). Although the changes taking place in members' constituencies have had a statistically significant and noticeable effect on party polarization, it alone does not explain all or even a majority of it. In part III of the book, I find that the lion's share of party polarization has come about as a consequence of the procedural divide between Democrats and Republicans in both the House and the Senate. To be sure, this procedural divide has roots in the growing partisanship of members' constituencies. These changes must interact with the legislative process for a clearer picture of party polarization to come into focus.

Chapter 7 examines the effect that increasing partisanship of members' constituencies has on the internal dynamics between members and their party leadership (arrow E in figure 3.2). As the party caucuses' constituencies have become more internally homogeneous, the rank and file members have been more willing to cede to their party leadership an increasing scope and use of powers so that they can entice, cajole, compel, or force party loyalty among the rank and file members. More polarized caucuses have led to more polarized party and committee leaderships in both chambers of Congress.