Switzerland is a semi-direct democratic federal republic. The federal legislative power is vested in the two chambers of the Federal Assembly: the National Council and the Council of States. The Federal Council holds the executive power and is composed of seven power-sharing Federal Councillors elected by the Federal Assembly. The judicial branch is headed by the Federal Supreme Court of Switzerland, whose judges are elected by the Federal Assembly. Switzerland has a tradition of direct democracy. For any change in the constitution, a referendum is mandatory (mandatory referendum); for any change in a law, a referendum can be requested (optional referendum). In addition, the people may present a constitutional popular initiative to introduce amendments to the federal constitution. The people also assume a role similar to the constitutional court, which does not exist, and thus act as the guardian of the rule of law. Cantonal and municipal politics vary in the different cantons, which may have different systems. Direct representation Switzerland features a system of government not seen in any other nation: direct representation, sometimes called half-direct democracy (this may be arguable, because theoretically, the sovereign of Switzerland is actually its entire electorate). Referendums on the most important laws have been used since the 1848 constitution. Amendments to the Federal Constitution of Switzerland, the joining of international organisations, or changes to federal laws that have no foundation in the constitution but will remain in force for more than one year must be approved by the majority of both the people and the cantons, a double majority. Any citizen may challenge a law that has been passed by parliament. If that person is able to gather 50,000 signatures against the law within 100 days, a national vote has to be scheduled where voters decide by a simple majority of the voters whether to accept or reject the law. Furthermore, any citizen may seek a decision on an amendment they want to make to the constitution. For such a federal popular initiative to be organised, the signatures of 100,000 voters must be collected within 18 months. Such a federal popular initiative is formulated as a precise new text (general proposal initiatives have been canceled in 2009) whose wording can no longer be changed by parliament and the government. After a successful signature gathering, the Federal Council may create a counterproposal to the proposed amendment and put it to vote on the same