I affirm that … [the states] would, under the plan of the convention, retain that authority [not delegated to the national] in the most absolute and unqualified sense; and that an attempt on the part of the national government to abridge them in the exercise of it, would be a violent assumption of power, unwarranted by any article or clause of its Constitution.
An entire consolidation of the States into one complete national sovereignty would imply an entire subordination of the parts; and whatever powers might remain in them, would be altogether dependent on the general will. But as the plan of the convention aims only at a partial union or consolidation, the State governments would clearly retain all the rights of sovereignty which they before had, and which were not, by that act, exclusively delegated to the United States. This exclusive delegation, or rather this alienation, of State sovereignty, would only exist in three cases: where the Constitution in express terms granted an exclusive authority to the Union; where it granted in one instance an authority to the Union, and in another prohibited the States from exercising the like authority; and where it granted an authority to the Union, to which a similar authority in the States would be absolutely and totally contradictory and repugnant. ….
… The rule that all authorities, of which the States are not explicitly divested in favor of the Union, remain with them in full vigor, is not a theoretical consequence of that division, but is clearly admitted by the whole tenor of the instrument which contains the articles of the proposed Constitution.
Source: Alexander Hamilton, Federalist No. 32, 2 January 1788.