Since it probably needs a Pre-1900s PoD, what is necessary to turn the United States of America into the "United Socialist States of America" ? One of the must-happen is a radicalization of the workers by the Gilded Age, prevent the monopoly breaks and worker's rights from being achieved by Roosevelt and the Progressives, and a worsened WWI. Maybe something like the Lusitania making the US enter in 1915, and the war ending in 1917 after the US bleeds hundreds of thousands of men in the trenches. Add in an incompetent diplomacy giving in to the French and British and the Americans will believe they bleed for sake of Anglo-French Imperialism. Then throw the Spanish Flu in America and a Post-War Depression. Is this the perfect storm already ? Or I still need more sauce ?