The islands of São Tomé and Príncipe were colonized by Portugal in the late 1400s as a site for sugar plantations worked by West African slaves and Portuguese overseers. The mixing of these elements over several centuries produced a creole national culture, which blended African and European traditions, languages, and economic culture. São Tomé and Príncipe achieved status as an overseas province of Portugal in 1951 and local autonomy in 1973. A nationalist group called the Comissao de Liberacao de São Tomé and Príncipe (Commission for the Liberation of São Tomé and Príncipe—CLSTP) formed in 1960, and in 1972 it reorganized itself as the Movimento de Liberacao de São Tomé and Príncipe (Movement for the Liberation of São Tomé and Príncipe—MLSTP) under the leadership of Dr. Manuel Pinto da Costa. Following a 1974 military coup in Lisbon, Portugal gave independence to all of its overseas colonies. In São Tomé and Príncipe, elections to a constituent assembly were held in July of 1975 and the MLSTP won all 16 seats. Pinto da Costa was elected as president. During the years 1976–85 he opened trade and diplomatic ties with Eastern Europe and accepted military advisors from the Soviet Union and Cuba. In spite of growing internal opposition, Pinto da Costa served three five-year terms as president.
In 1989 the MLSTP endorsed a plan for a new Constitution, which guaranteed multi-party elections, universal suffrage, and freedom of the press. Pinto da Costa agreed to step down from office and was replaced as MLSTP head by Dr. Carlos da Graca, who had returned from exile in Gabon. The party also merged with an opposition group, the Partido Social Democratica (PSD), to form the MLSTP-PSD. In the 1990 elections 72% of the population voted to accept a new Constitution. In the new system legislative power is vested in a national assembly, which comprises 55 members elected for four years. While executive powers are invested in the government, the president, who is elected to a term of five years, is nominally head of the armed forces and in charge of foreign affairs. The Constitution also limits the president to two successive terms. The government is headed by the prime minister, who has both executive and administrative duties.
The 1990 Constitution, however, suffers from a vague separation of powers, and contains two conflicting methods for the appointment of the prime minister. Under one clause,
the president retains the right to appoint the prime minister. Another clause states that the prime minister should be selected by the party that wins a majority in the elections to the National Assembly, held every four years. This structural contradiction has resulted in considerable political instability since 1991, and has been blamed for frequent government changes (seven since the introduction of a multi-party system), delays and/or blockage of many of the major political and economic decisions, including the privatization of public companies. The 1996–98 coalition government proposed a revision of the 1990 Constitution to clarify and curb the president's powers, which yielded no results. In January 2003, de Menezes dissolved and then revoked the decision to dissolve the National Assembly over proposed constitutional changes that would have weakened the presidency. A supreme court holds judicial authority and is responsible only to the National Assembly.