Keep the reformist faction from taking control of the National Executive Committee of the Socialist Party in 1912 would be a good start. The adoption of the reformist platform in 1912 caused the SPA to irrevocably split from the IWW and industrial unionism.
That disastrous weakening of the party would prove disastrous during WWI. State repression nearly destroyed the party in its entirety, and the change would have devastating consequences later.
Because of that, the Left faction was expelled from the party in 1919 over the formation of the Comintern. The new Communist Party was much easier for the Comintern to dominate, and it was set in a permanent antagonistic relationship with the Socialist Party.
This all left the party a shell of its self throughout the twenties. Even without it's former organizing strength or the logistical support of an industrial union, Norman Thomas got over 800,000 votes in the 1932 Presidential election, about 3.2 percent of the votes cast.