You are on to something but it seems that american exceptionalism was there from the very beginning and before it was a global superpower :
From wikipedia:
Jefferson envisaged America becoming the world's great "empire of liberty"--that is, the model for democracy and republicanism. He identified his nation as a beacon to the world, for, he said on departing the presidency in 1809, America was:
"Trusted with the destinies of this solitary republic of the world, the only monument of human rights, and the sole depository of the sacred fire of freedom and self-government, from hence it is to be lighted up in other regions of the earth, if other regions of the earth shall ever become susceptible of its benign influence."[37]
Off-topic: that wp [1] page states that american decline started at the end of the eighties. It makes an interesting read as I think the mass media put it around 9/11. Happened before 9/11 but that event crystallized that decline.
[1] http://en.wikipedia.org/wiki/American_exceptionalism
Edit:
1. The US declared independence in 1776 and were recognized in 1783. Is 30 years between this and Jefferson statement too much to say "it was there from the beginning ?"
2. What about 1600's and 1700's US religious people and their own beliefs when they established themselves in that new promised land ? How does it relate to "American ideology, "Americanism", based on liberty, egalitarianism, individualism, republicanism, populism and laissez-faire." (from the first paragraph of that wp page) ?