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.