In a Republic, our elected officials are supposed to govern according to
the law (with the Constitution trumping any laws that congress craps out
short of a Constitutional amendment, let alone any executive order).
Any law is ineffective unless enforced upon violation found. Our ultimate
finders are nine justices. Nine humans applying their version of what the
Constitution means. Hence the intense struggle to stack the court with
those who interpret the Court literally versus substantively depending on
circumstances.
Once the Supremes rule that an action is illegal, then the Justice
Department must enforce the ruling. But at the onset, the Supremes decide
which cases they will take. To elaborate on your apt tag line: when one
party controls the executive, the legislative, and the judicial branches of
our government, we no longer have a Republic; we have a tyranny with a
tyrant supported by partisans who may fairly be judged criminals by citizens.