Socialism

every time a socialist is elected there will be powers that cause their assassination and/or overthrow.

First and most importantly, you are correct about all of the above, and I am just giving a thought. I could at least see that one as being a spark to mobilize to revolution. Especially if the far-right conservatives/fascist groups took that as their "cue" to start "taking back America" via open killing other leftists (and liberals since they have been groomed to see liberals as being "communists").

Not super likely given actual history, but seems like the most likely situation out of the four problems given. Too many things would have to happen to make the "spark" catch instead of fizzle out. Just like the mass anger at Trump will regress "back to normal" the moment Dems likely win the White House after the next national election (even if the Dems do what they did after W and not actually undo anything).