With Justice Souter's recent retirement announcement, and the ensuing nomination of Sonia Sotomayor, it is worth revisiting Souter's ideological role in the Court. How would we broadly classify his ideology? How did it change over time? And how does his ideology relate to that of his contemporaries and his forebears?
Investigate the Ideological History of the Supreme Court yourself at http://scotusscores.com
Fortunately, there is a data-driven metric we can use to answer these questions. Political scientists Andrew Martin (Washington University) and Kevin Quinn (Harvard University) have developed numeric ideological estimates for each justice in each court term dating back to 1937. Negative numbers denote ideological liberals; higher positive numbers indicate a more conservative Justice.
TargetPoint Consulting has placed these scores into an interactive data visualization that mashes up a timeline, a bar graph and a heat map:
See the interactive data visualization here http://scotusscores.com
Justice Souter's Martin-Quinn scores have ranged from a high +0.97 in his first term on the court to a low of -1.7 in the 2003 term. Most recently, his score for the 2007 term was -1.5. Across his 18 terms as an Associate Justice, his average ideological score was a left-of-center -0.87.
Appointed by the first President Bush, he was generally expected to be a conservative voice on the Court. Within only four terms his scores ended up on the liberal side of the ledger, and his liberalism only continued to increase throughout the remainder of his time on the Court.
Interestingly, Bush nominated Souter to replace another conservative disappointment, Eisenhower appointee William J. Brennan, whose ideological drift away from conservatism was even more pronounced than Souter's (by Brennan's last term, with a rating of -3.7, he had become one of the most liberal judges in the 70 year history of the Martin-Quinn scores).
Looking separately at teach term Justice Souter served in, his recent liberalism is all the more stark. In his first two terms on the Court in 1990 and 1991, he was actually the most likely Justice to be the deciding vote on close cases, with a Martin-Quinn score that placed him directly in the ideological center of the Court. Following that, however, he shifted further and further to the left until the 2006 and 2007 terms in which his ideology (-1.478) effectively matched that of the liberal cohort of Justices Ginsburg (-1.505), Breyer (-1.634), and Stevens (-1.985).
You can find the full ideological data for all justices and courts going back to 1937 at http://scotusscores.com where you can investigate and interact with the data yourself.