In 1890, the SLP came under the leadership of the famous (and infamous) doctrinaire, Daniel De Leon, a lawyer how lectured at Columbia Law School. From that point to the present, the SLP has adhered to the form of orthodox Marxism known as DeLeonism. This caused De Leon's opponents, led by Morris Hillquit, to leave the SLP in 1901.
Hillquit's "Kangaroo" faction fused with Eugene Debs's Social Democratic Party and formed the Socialist Party of America. From that point forward, the SLP lived in the shadow of the much larger and popular Socialist Party. And later, the SLP lost even more footing when two other parties, the Communist Party and the Socialist Workers Party began capturing militant Marxists who would have otherwise joined the SLP.
Always critical of both the Soviet Union and of the Socialist Party's "reformism," the SLP has been isolated from the majority of the American Left, and that isolation seems to be ever-increasing. In 1976, the SLP ran its last Presidential race, and hasn't run many campaigns since then. They recently have been having trouble even funding their newspaper, The People.