Federalism gives the sovereign inherent authority to perpetuate itself. This is basic western philosophy of government. I mean, have you heard of martial law? Not in the Constitution either, but zero question of its legitimacy in times of existential threat to the federal power.
Hey if only downvotes changed facts. There are also the necessary and proper clause, police powers clause, general welfare clause, common defense clause, war powers clause, interstate commerce clause. You go read it.