Some point to the United States Code (USC) to claim that the United States is a federal corporation, and not a union of states as described in the original Constitution.

From Title 28, Part VI, Chapter 176, Subchapter A, Section 3002 of the USC:

(15) “United States” means—
(A) a Federal corporation;
(B) an agency, department, commission, board, or other entity of the United States; or
(C) an instrumentality of the United States.

If you look at the context of that definition, it becomes clear that it’s not saying that the United States is a federal corporation, but rather, it’s referring to federal corporations incorporated by the United States.

At the beginning of the section, it says: “As used in this chapter:”

Therefore, the reference to the “United States” as “a federal corporation” is only applicable to Title 28, Part VI, Chapter 176 of the United States Code.

Even within that limited context, it’s not referring to the United States as a federal corporation. If that was the intent, it would have been defined as “the United States, a Federal corporation.”

Looking at a different subchapter of the same chapter, namely, Subchapter D, Section 3306 (Remedies of the United States), (a):

“(1) avoidance of the transfer or obligation to the extent necessary to satisfy the debt to the United States;“

If the meaning of  “(A) a Federal corporation” is substituted, we get:

(1) avoidance of the transfer or obligation to the extent necessary to satisfy the debt to a Federal corporation.

Examples of United States federal corporations can be found here.

In my article, A Constitution for the United States of America or of the United States?, I show how the Founding Fathers who signed the Constitution drew no distinction between the two, despite the claim that the United States of America is a nation and the United States is a corporation.