The USP traces its origins back to the main leftist opposition faction within the Lanka Sama Samaja Party, the Vama Samasamaja.

Disagreements on areas of theory and policy (such as the analysis of Stalinism) with the CWI eventually led to a second split.

[3] One disagreement was over the leaders' support of the controversial Indo-Sri Lanka Accord, signed in 1987, which led to the deployment of the Indian Peace Keeping Force, in a document issued on 12 February 1988.

[5] The party opposed the Rajapaksa government's militarist solution to the country's civil war, holding the position that military victory in the Tamil-dominated areas would not solve the national question, with ill feelings and further violence likely to erupt as a result.

USP General Secretary, Siritunga Jayasuriya, is also quoted with saying that the government's military operation was a "war not against LTTE cadre, but against the Tamil people.