That's an extremely simplistic model, when in reality the way to fascism, is that the democracy fails to protect itself against it.
This happens easiest in new or flawed democracies, and USA is very much a flawed democracy. Where all the checks and balances don't work, when the executive branch is filled with corrupt traitors that don't give a shit about decency or democracy.
In a democracy the parlament can overthrow the government, and end its control with the equivalent of the executive branch.
AFAIK congress cannot overthrow a president for being unworthy or acting illegally.
Americans probably thought they were safe, because they have about 200 years of tradition for democracy, so they thought it was so ingrained that it couldn't fail, because the country is built on it, and the institutions would protect it.