Linda Greenhouse cites an article on judicial decision-making that notes that "judging at the level of the Supreme Court involves a complicated blend of legal, policy, and ideological considerations.” She continues:
“Judges’ decisions are a function of what they prefer to do, tempered by what they think they ought to do, but constrained by what they perceive is feasible to do.” By “feasible,” Professor Gibson means something deeper than “what they can get away with.” He means that judges operate within the public and private norms of the judicial role and of their own institution, constrained by the need to safeguard institutional legitimacy and by “the sociopolitical environment within which the institution is located.”Justices are not merely fungible but they also cannot (or, to simplify, do not) -- even in the obvious 5-4 rulings of which about a quarter are -- just act like politicians. Some cynically say they do, but this simply is not what occurs. This is not to say that in various controversial areas that the judges break down in predictable often liberal/conservative type ways, but even there, not all the time.
To the list of constraints, I would add the text of the constitutional provision or the statute at issue, along with the relevant precedents that guide the analysis.
And, they only go just so far, for various reasons, including the text, precedent, institutional realities etc. This perhaps is part of the reason why the public still respects them more than "politicians," even with the Gingrich type comments.