Social Democratic Party of Sierra

The Social Democratic Party of Sierra (commonly known as the Sierra SDP or Sierra Social Democrats) is the branch of the Social Democratic Party of Enderron operating in Sierra Prefecture. The party is the one of the three major political parties in Sierran politics, the others being the Liberal Party of Sierra and the Sierra National Party. The current party leader is Premier Cathy Rathburn, since 2010. The party has been the governing party of Sierra since 2012.

The Sierra SDP largely remained in opposition from 1946 to the 1990s, only briefly forming a minority government from 1975 to 1976 under Premier Charlie Hartwell. The party won its first prefectural election in 1992. The party was a junior coalition partner with the National Party from 1998 to 2000, and with the Liberal Party from 2004 to 2008. The party has elected three governors to office: Edgardo Castaneda (1968–72), Pat Jessop (1992–96) and Colin Bartha (2016–20).

At the federal level, the Sierra SDP last won Sierra's sole seat in the Enderronian Parliament in 1994, and has not represented the prefecture since 1995.

The Social Democrats are strongest in their urban base of Los Prados, the only major urban area in Sierra, as well as some mining towns scattered throughout the prefecture.