A Union of Sovereign States

Madison has often been called the Father of the Constitution. Thomas Jefferson also said that the federal government was a voluntary association of states.

"Each State, in ratifying the Constitution, is considered as
a sovereign body, independent of all others, and only to be
bound by its own voluntary act. In this relation, then, the
new Constitution will, if established, be a FEDERAL, and not a
NATIONAL constitution."

-- James Madison (Federalist No. 39, 1788)

This is why, for example, the national popular vote is irrelevant in a presidential election, which is actually separate elections in the 50 states and the District of Columbia.