Zimbabwe is a republic with a presidential system of government. The semi-presidential system was abolished with the adoption of a new constitution after a referendum in March 2013. Under the constitutional changes in 2005, an upper chamber, the Senate, was reinstated. The House of Assembly is the lower chamber of Parliament. Former President Robert Mugabe's Zimbabwe African National Union – Patriotic Front (commonly abbreviated ZANU–PF) has been the dominant political party in Zimbabwe since independence.