A lot of people here talk about the stigma of the word socialism in the US as though it was the natural platonic state of an American to view socialism with suspicion and disdain. Yet this kind of overlooks the fact that such suspicion was something that was actively cultivated and constructed over a century of political suppression and ideological warfare. There was a time when the US had one of the fastest growing Socialist Party in the world. Without WW1, or at least Americas entry into it, which was arguably one of the main factors in disrupting American Socialism due to the internal splits it caused (pro and anti war and then pro and anti Communist) and the external repression brought about by the Sedition Act and First Red Scare, they could well have continued to gain support.
If you want an interesting option for a Socialist President that hasn't been mentioned yet perhaps I could recommend
Horace Greeley? He was, amongst other things, a
Fourierist, who were an early utopian socialist movement, and owned the newspaper that Karl Marx was a correspondent in. He stood as the Liberal Republican presidential candidate in
1872 but both lost in a landslide against grant and died before the EC voted. He was, however, very active in US politics throughout his life so it isn't impossible to imagine a scenario where he gets a nomination at a previous date.