How the powers are divided between president and prime minister can vary greatly between countries. In France, for example, in case of cohabitation whereas the president and the prime minister come from opposing parties, the president is responsible for foreign policy and the prime minister for domestic policy2. In this case, the division of powers between the prime minister and the president is not explicitly stated in the constitution, but has evolved as a political convention. In Finland, by contrast, this particular aspect of the separation of powers is explicitly stated in the constitution: "foreign policy is led by the president in cooperation with the cabinet".
Cohabitation
Semi-presidential systems may sometimes experience periods in which the President and the Prime Minister are from differing and opposing political parties. This is called "cohabitation", a term which originated in France when the situation first arose in the 1980s. In most cases, cohabitation results from a system in which the two executives are not elected at the same time or for the same term. For example, in 1981, France elected both a Socialist president and legislature, which yielded a socialist premier. But whereas the president's term of office was for seven years, the National Assembly only served for five. When, in the 1986 legislative election, the French people elected a right-center Assembly, Socialist President Mitterand was forced into "cohabitation" with a rightist premier.
Cohabitation can create an effective system of checks and balances or a period of bitter and tense stonewalling, depending on the attitudes of the two leaders, the ideologies of their parties, or the demands of their constituencies. As a typical example, Sri Lankan politics for several years witnessed a bitter struggle between the President and the Prime Minister, belonging to different parties and elected separately, over the negotiations with the LTTE to resolve the longstanding civil war.
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