Your challenge is, if you choose to accept it, to make the 20th century the worst on the United States as feasibly possible, with no nuclear attacks. Personally, I find those one hundred years to be full of opportunities, such as a longer, harder WWII, a bloodier Cold War, a longer Great Depression, etc.
Since this is the second time I've tried posting a WI like this, I'm going to clarify it a bit. By "worst century possible," I do not mean having the US collapse or Sealion be successful or other ASB events. What I do mean by it is that, at least in my opinion, there are a lot of incidences throughout the 20th century that could have gone a lot worse for the USA. Think the "Times of Trouble" timeline. Thus, this WI should be focused on the States having the worst luck possible, but still a world power.