As we already touched on briefly, the people of the states, who delegated that power in the first place.
“We the People” are the sovereign in the system. Not the Congress, not the president, not the courts. The people have the final say. We the people had already created sovereign political societies – states – prior to the ratification of the Constitution. The people only transferred limited powers to the general government and did not dissolve their original political societies. The state governments retain all power except those delegated. It only makes sense that the people of the states retain the authority to determine – in the final instance – the extent of the power they themselves delegated. The proper authority for this determination rests in the state legislatures – the representatives of the people within their original political society.
Many of the ratifying documents made this clear. Take New York’s, for example.
“That the powers of government may be reassumed by the people whensoever it shall become necessary to their happiness; that every power, jurisdiction, and right, which is not by the said Constitution clearly delegated to the Congress of the United States, or the departments of the government thereof, remains to the people of the several states, or to their respective state governments, to whom they may have granted the same.”