Below we show the polarization of the Political Parties for the 1879 through 2014 period (46th to 113th Congresses). Polarization is measured by the distance between the means of the Democrat and Republican Parties on the first (Liberal vs. Conservative) DW-NOMINATE dimension. Polarization is now at a Post-Reconstruction high in both the House and Senate.
Below are the Party means for the House on the first DW-NOMINATE dimension. For the Democrats we show the means for the Northern and Southern wings of the Party (We use the CQ definition of South; the 11 States of the Confederacy plus Kentucky and Oklahoma). In the past three Congresses the difference between the Northern and Southern Democrats has disappeared.
Below are the corresponding Party means for the Senate on the first DW-NOMINATE dimension. The pattern is essentially the same as the House. However, the Southern Democratic Senators as a group tend to be more moderate than their Northern counterparts.
Below are the Party mean graphs for the House and second for the second DW-NOMINATE dimension. This dimension usually picks up regional differences between the two major parties. Before the Civil War the second dimension picked up the North vs. South division on Slavery. In the Post Reconstruction period the second dimension picked up regional differences on soft vs. hard Money (bimetalism, gold and silver) and beginning in the late 1930s Civil Rights. In the past 20 years the second dimension has faded in importance and issues that used to divide the parties internally — e.g., gun control, abortion — now load almost entirely on the first dimension. This is explored in detail in Keith T. Poole and Howard Rosenthal (2007) Ideology and Congress. Note that in the figures below the parties have almost converged on the second DW-NOMINATE dimension. Voting in Congress is almost entirely one-dimensional. The first dimension now accounts for over 93-94 percent of the roll call votes.