Technically, when you cross one US state into another, you are traveling internationally.
We could say one is also traveling intranationally as well given the States have individual (the several States) as well as an aggregate capacity known was the United States of America.
A symbol of the aggregate capacity of the States is the U.S. Constitution .... a compact (treaty) ... and a charter establishing an international public corporation styled government of the United States.
A federal government (as opposed to a national government).
Probably the closest thing to a "passport" within the territorial United States would be a driver's license.