There would be several ways, militarily would be one, possibly the one involved with the least 'difficulty', however, a primarily militarily established dictatorship, which most likely would be militarily enforced, is quite difficult to sustain over the long run due to resistance or new dictators. The majority of African dictatorships should show that. A dictatorship, such as made by Hitler, utilized a lot of thought process and some time, while including the majority of the population as supporters (or at least supporting in one way or another), which is way more 'effective'. However, a militarily established dictatorship could arise in the US or in fact in an country, command of the armed forces, as it mostly is the case with African dictators, could establish the such. How popular such a regime would be is another question though.
The only way I can see a dictator emerging in the United States would be like a Hitler scenario. He/she gains his power and legitimacy from the people/public, and when "elected" he/she begins to slowly reside rights and assert absolute power, which would be grounded from the people. An authoritarian dictatorship is most unlikely because there's no way the people will approve of a dictator where his power resides in the elite.