A Visual History of Senate Polarization from 1967 to 2010
(Maximize the video and set the quality to 720p HD for best viewing. The video is also available for download from here.)
Each senator is marked by a two-letter state code and is color coded by party membership (Democrat, Republican, Independent). The top grey bar is the distance between the mean Republican and the mean Democrat (the standard measure of partisan polarization). The size of each party’s coalition is displayed at the end of the bars. The lower grey bar is the gridlock interval, which is measured as the distance between the filibuster pivots (the 34th and 66th most conservative senators before the Senate rule change in 1975 and the 41st and 60th thereafter). The ‘M’ imposed on top of the bar marks the position of the median senator.
The ideological estimates are constructed by scaling senate voting records with dynamic optimal classification. Keith Poole’s paper on optimal classification can be found here, and the paper which explains how to extend the method to smooth legislator ideal points over time can be found here.
Ideological Cartography 
Very interesting viewing. What was the “2nd Dimension?”
The second dimension usually measures positions that are orthogonal the main liberal-conservative dimension at a given time. During the 60’s and 70’s the second dimension pretty clearly picked up voting on civil rights issues. There is evidence that the second dimension picked up cultural issues like a abortion during the 80’s and 90’s, but these issues had largely folded down into the liberal-conservative dimension by the start of the last decade. There doesn’t seem to be a consistent pattern to the issues picked up the second dimension since then. It seems vary from one congress to the next. In the 111th Senate, many of votes that scale to the second dimension deal with earmark reform. Keith Poole has an nice application on his site which you can use to see which votes scale to which dimensions: http://voteview.com/senvotes.asp . The one thing that is clear is that second dimension is far less important in predicting vote outcomes now than it had been in the past.