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Transition

Juan Carlos's accession met with relatively little parliamentary opposition. Some members of the Movimiento Nacional voted against recognizing him, and even more voted against the 1976 Law for Political Reform. But a majority of Movimiento members supported both measures. Juan Carlos quickly instituted reforms, to the great displeasure of Falangist and conservative (monarchist) elements, especially in the military, who had expected him to maintain the authoritarian state.

In July 1976, Juan Carlos dismissed prime minister Carlos Arias Navarro, who had been attempting to continue Francoist policies in the face of the King's attempts at democratization. He instead appointed Adolfo Suárez, a former leader of the Movimiento Nacional, as prime minister.

Further legitimacy was restored to Juan Carlos's position on 14 May 1977, when his father (whom many monarchists had recognized as the legitimate, exiled King of Spain during the Franco era) formally renounced his claim to the throne and recognized his son as the sole head of the Spanish Royal House, transferring to him the historical heritage of the Spanish monarchy, thus making Juan Carlos both de facto and de jure king in the eyes of the traditional monarchists.

On 20 May 1977, the leader of the only recently legalized Spanish Socialist Workers' Party (PSOE), Felipe González, accompanied by Javier Solana, visited Juan Carlos in the Zarzuela Palace. The event represented a key endorsement of the monarchy from Spain's political left, who had been historically republican. Left-wing support for the monarchy had grown when the Communist Party of Spain was legalized on 9 April 1977, a move Juan Carlos had pressed for, despite enormous right-wing military opposition at that time, during the Cold War.

On 15 June 1977, Spain held its first post-Franco democratic elections. Juan Carlos had played a role as middleman in order to channel $10 million from the Shah of Iran to Adolfo Suárez's election campaign, reportedly asking the Shah for the money to "save Spain from Marxism". Suárez went on to win the election and become the first democratically elected leader of the new regime.

In 1978, the government promulgated a new constitution that acknowledged Juan Carlos as rightful heir of the Spanish dynasty and king; specifically, Title II, Section 57 asserted Juan Carlos's right to the throne of Spain by dynastic succession in the Bourbon tradition, as "the legitimate heir of the historic dynasty" rather than as the designated successor of Franco. The Constitution was passed by the democratically elected Constituent Cortes, ratified by the people in a referendum (6 December) and then signed into law by the King before a solemn meeting of the Cortes.