Their are three things you really have to do;
1. Prevent the laws that basically all but destroyed the labour and Worker's movements in the early 20th century..
2. Not have Socialism linked to rabid isolationism and pacifism, America may have at the time not been interested in the rest of the world,
but their stances hurt their support in the end and were part of the reason the government actively worked to suppress them, since they
saw them as a threat to the War Effort in the First World War.
3. Give it different leadership over time, The Socialist Party had some good leadership, but over time it's leadership has sucked greatly,
which is itself part of the reason their are now a dozen different parties all descended from the original Socialist Party.