The monarchists did not have any real power until the last dying days of the Weimar Republic - which violence between the communists (who had split from the SPD after the failed revolutions) and nazis had accelerated dramatically. (Along with the collapse of international finance and easy American loans).
It was the collapse of the three parties that led to the rise of the militarists (not necessarily monarchists) and ultimately to Hitler.
Sources - Richard Evan’s three volume history of nazi germany is great.
