Using roll call votes from the 110th Senate through the end of last year, I have constructed a network diagram based on maximum similarities between Senators’ voting records. Essentially, distances were calculated by assigning a 1 to yes votes, and a 0 to no votes, and finding the difference between each pair of Senators on each possible roll call vote. Thus, two Senators who vote identically have a distance of 0, while two Senators who vote completely opposite ways have a distance equal to the total number of roll calls. Based on these distances, I constructed a network diagram linking each Senator to their two most-similarly-voting counterparts. I also colored each vertex according to how similar each Senator is to “all Republicans” and “all Democrats” collectively. The result revealed the highly polarized nature of the Senate: there is only a single strand linking Republicans to Democrats:
11oth Senate Roll Call Network Diagram [pdf]
I then decided to reduce the number of connections to only the single closest match for each Senator, and found something interesting that you will hear only rarely from the media: Senators Clinton and Obama are each others’ closest match, based, at least, on roll call votes in the 110th Senate through the end of 2007. This would seem to indicate that the wide disparities perceived between them in the eyes of the media and the public have little to do with actual policy/ideological divides, but rather that personality and framing (and possibly demographics) are making up the bulk of voting preferences in many Americans’ minds.
110th Senate Roll Call Isolated Networks [pdf]
I was aware, to some extent, of the constructed, rather than actual, nature of the differences between the two Democratic competitors, but to see the roll call evidence fall out so starkly was surprising.