"Actually, Sonny wanted the term of copyright protection to last forever. I am informed by staff that such a change would violate the Constitution. … As you know, there is also [then-MPAA president] Jack Valenti’s proposal for term to last forever less one day. Perhaps the Committee may look at that next Congress."
Mary Bono
The line in the Constitution that justifies the existence of copyright law in the U.S. specifically requires that copyright last “for limited times.” The “proposal for term to last forever less one day” Mary Bono referenced basically meant that Congress should extend copyright every time anything ever gets too close to entering the public domain.