American school children often take a civics course around 6th grade where they are taught a basic theory of how democracy in general, and the American constitution in particular, work. The good news is that most students forget what they learn by the time they graduate--and the reason that's good is because almost everything they were taught is wrong. (For instance, they are taught an obsolete theory of voter behavior.)
When it comes to the Supreme Court, they are usually told that the justices enforce the Constitution. Justices do not make the rules but rather ensure that the rules conform to and comply with the meta-rules.
Of course, in practice, we all recognize this is mostly, if not entirely, bullshit. This is one reason why the death of a Supreme Court justice is so disturbing to us. (The other reason is that we gain social benefits by pretending we are more upset than we are, as this impresses others in our peer group by signaling a high degree of intelligence, loyalty, and civic-mindedness.) We recognize that justices have political bents, and in general they reverse engineer a theory of legal and constitutional interpretation--a theory of the meta-rules--such that the Constitution tends to require what they independently prefer and forbids what they independently hate. (I'm not saying they are insincere, but rather self-deceived.)
How else could it be that the all the left-wing justices think the Constitution is left-wing and all the right-wingers think it's right-wing? What an amazing coincidence! You might say that some theories of Constitutional interpretation explicitly say that the content of the law is somehow directly attached to morality and justice, and so of course justices must interpret the law that way. But then notice that many justices are legal positivists, original intent theorists, or original meaning theorists, and yet somehow they also tend to think the constitution usually implies what they independently would want the law to be. Hmmm.
Update: I should be clearer here. My post may exaggerate the strength of the effect. I'm not saying that justices will always vote their politics. The situation is more akin to what some historians of philosophy do. Imagine a bunch of them like Adam Smith. The left-wing theorists tend to portray Smith as left-wing, and the right-wing theorists as conservative. The libertarians think he's libertarian. Here, however, they do face constraints, because you can only bullshit or stretch the text so much. The same goes for justices. They do tend to push past cases, the law, the meta-laws, and so on, in their favored direction, but they don't outright lie. The law does impose a constraint.
Note that my personal position is that justices are prima facie morally required to lie about the content of the Constitution in order to promote justice. For instance, if you have a court case about slavery, and it turns out that on the correct theory of interpretation the constitution allows slavery, prima facie, you must nevertheless say that it does not. (I say "prima facie" because issues about compliance, backlash, future opportunities to thwart injustice, and so on, make it more complicated than this. See the linked book.)
The reality is that the Supreme Court is a "super legislature". See this paper by Brian Leiter:
I propose to defend and explore three claims in this Essay. First, there is very little actual “law” in federal constitutional law in the United States, especially with respect to cases that end up at the Supreme Court. There, the Court operates as a kind of super-legislature, albeit one with limited jurisdiction. The jurisdiction is limited in two important ways: first, the Court can only pass on issues that are brought before it; and second, the Court is constrained, to some extent, by its past decisions and by constitutional and legislative texts. The problem, however, is that those constraints underdetermine the Court’s decisions in most cases, so the Court essentially makes its final choice among the legally viable options based on the moral and political values of the Justices, and not simply on the basis of legally binding standards...
The US has in effect three legislature houses, with the Supreme Court functioning as the highest house, towering over the Senate and House of Representatives. Justices get jobs for life, are difficult to remove, aren't beholden to voters or anyone else, and have far more voting power than any senator. Of course these political battles are high stakes.
American politics is now bound to be even more toxic and obnoxious.