In terms of actual rulings, is it possible to put a figure on how much of an improvement Sonia Sotomayor is likely to be over George Bush's two Supreme Court nominees? Even though predicting the voting tendencies of Supreme Court justices is famed for being difficult, the answer appears to be yes. The best guess is that Sotomayor will rule with progressives about 25-30% more often than a Bush nominee would have done.
My reason for this is taken primarily from the recent voting history of David Souter, who Sonia Sotomayor is replacing. The numbers that follow (taken from Scotus Wiki) depict the widest range of disagreement Souter had with other members of the Supreme Court from 2003-2008.
Toward the end of his term, Souter was most closely aligned with the female members of the Supreme Court. He consistently had very little disagreement with Ginsburg and O'Connor. So, it makes sense that he is being replaced by a woman.
Souter's level of disagreement with the most right-wing member of the court was stable, hovering just under 20 cases a year. This represents 25-30% of all cases.
If we assume that Sotomayor will continue in Souter's footsteps and rule more like Ginsberg than any other justice (which is more likely that Sotomayor being another Stephens or a swing vote like Kennedy), and that a third Bush nominee would have rulings like Alito, Roberts, Scalia or Thomas (which is likely given the similarity between the two Bush nominees and the two right-wing nominees who preceded them), then it seems likely that Sotomayor will side with progressives on about 19 or 20 more cases a year, or 25-30% of all cases, than a third Bush Justice would have done.
It is a crude estimate, but I believe one that will prove to be fairly accurate. Over the course of her career, having Sotomayor on the court instead of a third Bush nominee will end up being worth about 400-500 different rulings on cases, assuming that she stays on the court about 20-25 years. With about 20% of all rulings being 5-4 these days, that could mean up to 100 Supreme Court cases swung in the direction of progressives by this pick.
As such, in all likelihood, among Obama administration actions this year, nominating Sonia Sotomayor for Supreme Court nomination will be rivaled only by health care reform and climate change legislation in terms of long-term impact.
