Orin Kerr thinks that labeling Souter a "Yankee Republican" is ridiculous:
...consider the fact that the two Justices on the current Court who vote most frequently with each other are often Justice Souter and Justice Ginsburg. Looking at the current Supreme Court Term, for example, the Souter/Ginsburg pairing is the most common: They have fully agreed with each other 88% of the time. The next closest pairings are Scalia/Roberts at 83%, Roberts/Alito at 81%, and Thomas/Scalia at 79%.
I think it is generally recognized that Justice Ginsburg is not a Yankee Republican, and that she would not have been a Republican if the GOP had not become more conservative. Everyone pretty much agrees that Justice Ginsburg is very much a Democrat and at least somewhere on the left. But if the Souter/Ginsburg pairing is the closest pairing on the Court, closer than Thomas/Scalia, then isn't it a little strange to say that one is a liberal Democrat but the other is a Yankee Republican who only "seems" liberal?
I meant the term Yankee Republican primarily as a cultural moniker, but it's also true, I think, that its current incarnation is almost certainly closer to the Ginsburg view of the world than that of the current GOP or Thomas and Scalia. It is no accident that New England has pioneered marriage equality. I will grant one caveat, of course, and many readers noted it: Kelo.