Federalism requires at least two tiers of government endowed with a sovereign power on certain matters, a non-unilaterally amendable written constitution with a horizontal and vertical division of power, a representation mechanism of self-rule and shared rule, and an umpire to settle conflicts between the various tiers of government. These essential elements of federalism make constitutionalism an essential condition for its operation, at least according to conventional federal theory. Whether it is in the United States, Germany, Australia, Switzerland, Canada, or any other federal state, it is only through constitutionalism that federalism can perform its functions, deliver its promises, and live up to its expectations.
But can federalism operate without constitutionalism? Full article here