Assume that the US doesn't join WWI*. Without the OTL backlash from opposing American involvement in the war, would the SPA continue to grow? Or would it still be destroyed by infighting and government repression during the Red Scare?
*Entente and Central Powers come to some sort of truce in 1918, Russia still collapses, France and Germany trade colonies for Alsace-Lorraine.
*What* Red Scare? There were two bases for the Red Scare: (1) During the war, the belief that Bolshevik Russia was in league with Germany, a nation with which the US was at war. (2) After the war, the fear of Bolshevik Russian expansionism. Neither of these will exist in a scenario where (besides Germany not being at war with the US) Germany wins the war in the East (and gets a satisfactory peace in the West--though I am very dubious that the kind of tradeoff you envision could actually be arrived at, I'll assume it will be). For in such a situation Germany will quickly crush Bolshevik Russia and replace it with a puppet government like Skoropadski's in Ukraine.
So I don't see much of a Red Scare in the US against the Socialist Party, though of course there was plenty of hatred of the IWW even before the war.
And what may be the most important consequence for the Socialist Party, the left wing will probably not split off to form a Communist Party. This split had devastating consequences for the Socialists in OTL. There would still be infighting between the left and right wings of the party, but before 1919 the two wings did manage to coexist in the same party. Without a successful (in terms of survival) Soviet Russia and the formation of the Comintern, they would probably continue to do so.
This is not to say that I expect the Socialists to become a major party. I think they reached their peak in 1912. Eventually, the desire not to "waste" one's vote becomes fatal for minor parties in the US. (Incidentally, the *immediate* effect of the US entering the war was actually to increase the votes for Socialists, because voting Socialist was sometimes the only way you could protest the war. Hence Morris Hillquit's big vote in the 1917 New York mayoralty election and Victor Berger's strong showing in the 1918 Wisconsin special election for the US Senate, even while he was under indictment. No doubt a lot of German Americans who were not Socialists voted for Berger to protest the war.)