One of the better questions I've seen on here in a while. There is some debate on whether or not the American Empire is, in fact in decline, and surprisingly, some debate about whether or not it is an empire. In general, I find that Americans do not use the term empire that much in reflecting on their nation.
It is becoming easier to argue that we are in decline, however, and the path America seems to be following on that decline closely resembles that of other empires, most notably that of Rome and to some degree the Spanish Empire. The main similarity to the Roman Empire is that the US is militarily overextended and heavily in debt. Rome on the other hand was also besieged by invaders at the end of its decline, and that seems highly unlikely to reoccur in America. Spain, like America, was resource dependent, and that dependence both led the empire to become overextended and created an economy and lifestyle that were unsustainable. In Spain's case, that resource was gold, in America's case, I would argue it is oil.
The British Empire, on the other hand, also experienced decline because of military overextension and economic insolvency, but their empire was structured as a colonial one, and nationalism in their various colonies played a large role in their eventual independence, and the existence of the British Empire required they hold on to those colonies. America, by contrast, built a physical empire based on settlement and expansion rather than colonies, but has used its economic influence to in effect extend the same degree of imperial control. America's influence, as it currently stands, is also unsustainable.