Ethnic democracies are sustainable only if the two conflicting constitutional principles defining them as such – ethno-nationalism and liberal democracy – are successfully mediated by a third principle, of whatever kind. This is demonstrated through a comparative examination of inter-war Poland (1918–39), where ethnic democracy, while written into the constitution, never really took hold, and Israel (within its pre-1967 borders), where ethnic democracy was stable for thirty-five years (1966–2000) but may now be eroding. In both cases the fate of ethnic democracy is shown to be determined by the existence, lack, or decline of the mediating third principle.