Nothing, until the Nazi tries to take an illegal action to achieve their goals. Then you arrest them.
Now, seeking a change in the law is rarely illegal; even if it were (and the U.S. Constitution has a few things to say about equality), the law making it illegal could itself be changed. So it'd be possible to erode the rights of Jews over time – or Nazis, for that matter.
This kind of situation was envisaged by the Founding Fathers of the USA, but they had faith in the ability of the marketplace of ideas to resist such "errors in opinion" while there remained the capacity to use free speech to reason against them. However, if reasonable people agree that it's a good idea to discriminate, all bets are off.
Nothing, until the Nazi tries to take an illegal action to achieve their goals. Then you arrest them.
Now, seeking a change in the law is rarely illegal; even if it were (and the U.S. Constitution has a few things to say about equality), the law making it illegal could itself be changed. So it'd be possible to erode the rights of Jews over time – or Nazis, for that matter.
This kind of situation was envisaged by the Founding Fathers of the USA, but they had faith in the ability of the marketplace of ideas to resist such "errors in opinion" while there remained the capacity to use free speech to reason against them. However, if reasonable people agree that it's a good idea to discriminate, all bets are off.