With any POD you want, have the United States be a unitary republic, with no principle of federalism.
Simple, have the United States only comprise New England and New York.
If you mean the entirety of the United States than it's very close to ASB; Federalism is itself at the heart of America and really large countries do not work as Unitary states, their's a reason that four of the worlds five largest countries are Federations.