An oft-asserted claim is that the U.S. Supreme Court is ideological split between liberal or "leftist" (which appears to mean just "very liberal") and conservative factions, with Mr. Kennedy holding the balance (and before him, it was Ms. O'Connor). In other words, justices Souter, Stevens, Breyer, and Ginsburg are presumed to vote "liberal" while justices Scalia, Thomas, Roberts and Alito are presumed to vote "conservative."
My hypothesis is that this split is not ideological, but partisan. The liberal judges vote along the lines approved by the Democratic party (yes, I know Stevens and Souter were appointed by Republicans) while the conservatives vote along the lines approved by the Republican party. I believe this is true at least regarding the "hot button" issues of gun rights, rights of the accused, takings, the president's war powers, affirmative action and abortion (and maybe not so much on other issues).
I realize I need evidence to support this claim. However, consider the recent gun rights decision of D.C. v Heller. The "liberals" voted against striking down a D.C. gun control law and the "conservatives" voted for striking it down, with Mr. Kennedy voting with the conservatives. The "liberal" justices are much more likely to strike down laws that challenge the first, fourth and fifth amendments (except when it comes to takings....see the Kelo decision of a few years ago), and the "conservatives" are much more likely to sustain such laws. Yet these are all "rights" guaranteed in the Bill of Rights. Of course, when it comes to the second amendment, there is much linguistic confusion as to what that amendment actually guarantees. Still, if one reads Justice Stevens's dissent in Heller, even he acknowledges that gun ownership is an individual right (he might have been doing this as a tactic to sow the seeds of future qualifications on Heller).
I suspect that when future gun control cases come up that involve state laws (the D.C. law was technically a federal law), we will see "liberal" justices, who normally favor "incorporating" the Bill of Rights to apply to the states, will vote against such incorporation while the "conservative" justices will vote for such incorporation even though they generally dislike "incorporating" the bill of rights to apply to the states.
My hypothesis, then, is that at least on the controversial issues, the justices vote for constituencies and not for ideological preference.