David Bell well makes the point that for as long as the United States has been a superpower, people have been convinced that it is in decline, because of (choose as many as you like) laziness, moral turpitude, lack of discipline:
What the long history of American “declinism”—as opposed to America’s actual possible decline—suggests is that these anxieties have an existence of their own that is quite distinct from the actual geopolitical position of our country; that they arise as much from something deeply rooted in the collective psyche of our chattering classes as from sober political and economic analyses.
Bell goes back to the 1950s for his history of decline. I’d push it back further, into the 19th century at least.
UPDATE: It occurred to me later that such pronouncements of decline are a symptom of Very Serious People disease and are usually twinned with stern and moralistic remedies.
Hat tip, Ralph Luker