Interactive graphic by Christopher Ingraham. Data from Voteview.com. Original concept by Brian Resnick, Brian McGill and Ella Krivitchenko of the National Journal.

The National Journal graphic offered a unique way of looking at party unity scores for the 113th Congress: Party unity % on a y-axis, and a straightforward rank within each party along a split x-axis. The graphic showed that only 20 Republicans and 39 Democrats had party unity scores less than 90%, indicating a high degree of partisanship and polarization.

So what did previous Congresses look like? Voteview.com provides party unity scores going back to the first Congress. I limited myself to the 35th, since prior to that large numbers of non-Democratic or Republican party members (Whigs, etc.) make comparisons less useful. I did have to omit records of approximately 300 non-Democratic or Republican members from my subset, representing about 1% of the 30,000 or so records examined.

In general, more-unified Congresses have a narrower "stem" and flatter "flare" at the top of the graph, while less-unified Congresses have a wider stem. I'd caution against reading too much into the general shapes, however, since these are partially influenced by the number of representatives in each party in any given session. The data do suggest that our era is relatively unique in the level of party unity within Congress. The only other time period posting similar unity scores is the turn of the 20th century. Other eras are notable for lopsided unity rates. Immediately following the civil war, Democrats showed a much more unified front than their Republican colleagues.