The way an American would phrase it is:

To have a career as a politician in Germany you mainly have to impress the party bosses.

Not completely. If I live in Berlin and want to be elected into the Bundestag for the SPD I want to get a high place of the SPD list allocated in the Berlin SPD party convention. The head of the SPD in Berlin is the person got head because they have a majority of the Berlin SPD behind them, but their power over the convention isn't absolute. It's like the power the Nancy Pelosi has over democratic US congressman.

To have a career as a politician in the US you mainly have to impress your constituents.

Yes, constituents weighted by the amount of political donations that they can give.