People, were talking the late 1700s here. Grammar is not what it used to be.
The whole idea with that phrase was that the Constitution would be "closer to being finished, more complete, more fully formed" than the wonky, loosey-goosey Articles of Confederation.
The Constitution, in the eyes of the Framers, made the United States of America closer to a real country rather than a conglomeration of individual states (ie, their own countries).