Coat of arms of the Kingdom of Croatia, Slavonia and Dalmatia on the Croatian Parliament building
The Sabor, in its various forms, has represented the identity and opinions of Croats from the diets of the 9th century nobility to the modern parliament. The oldest Sabor whose records are preserved was held in Zagreb on 19 April 1273 as the Congregatio Regni totius Sclavonie generalis or Universitas nobilium Regni Sclavoniae (General diet of the entire kingdom of Slavonia or Community of the nobility of the kingdom of Slavonia). Its decisions had legislative power. The 1527 Parliament decision was a decisive event of fundamental importance for the extension and confirmation of Croatian statehood, as described by the Constitution of Croatia. The parliament freely chose Ferdinand I of the House of Habsburg as the new ruler of Croatia, after centuries of Croatian personal union with Hungary. Following the entry into the Habsburg Monarchy, the Sabor became a regular noble diet, and its official title gradually stabilised by 1558 to the Parliament of the Kingdom of Croatia and Slavonia. Since 1681 it has been formally styled as the Congregatio Regnorum Croatiae, Dalmatiae et Slavoniae or Generalis Congregatio dominorum statuum et ordinum Regni (Diet of the Kingdom of Croatia, Dalmatia and Slavonia or General Diet of the Estates of the Realm). In 1712, the Sabor once again invoked its prerogative to select the ruler, supporting what later became the Pragmatic Sanction of 1713 and electing Maria Theresa of Austria as monarch. This event is also specified by the Constitution of Croatia as a part of the foundation of unbroken Croatian statehood from the Middle Ages to the present.
In 1848 first modern Diet with the elected representatives was summoned (even high nobility and high dignitaries of the Catholic and Orthodox church remained ex officio members). The Sabor operated as the legislative authority during the existence of the Kingdom of Croatia-Slavonia (1848/1868 - 1918). The events of 1848 in Europe and in the Austrian Empire represent a watershed in Croatian society and politics, given their linkage to the Croatian national revival that strongly influenced and significantly shaped political and social events in Croatia from that point onwards to the end of the 20th century. At the time, the Sabor advocated the implicit severance of ties with the Kingdom of Hungary, emphasizing links to other South Slavic lands within the empire. A period of neo-absolutism was followed by the Austro-Hungarian Compromise of 1867 and Croatian–Hungarian Settlement, recognizing the limited independence of Croatia, together with reinvigorated claims of uninterrupted Croatian statehood. Two political parties that evolved in the 1860s and contributed significantly to this sentiment were the Party of Rights (1861–1929) and the People's Party. They were opposed by the National Constitutional Party that was in power for most of the period between the 1860s and 1918, which advocated closer ties between Croatia and Hungary. Another significant party formed in this era was the Serb People's Independent Party, which would later form the Croat-Serb Coalition with the Party of Rights and other Croat and Serb parties. This Coalition ruled Croatia between 1903 and 1918. The Croatian Peasant Party (HSS), established in 1904 and led by Stjepan Radić, advocated Croatian autonomy but achieved only moderate gains by 1918. In the Kingdom of Dalmatia, two major parties were the People's Party, a branch of the People's Party active in the Kingdom of Croatia-Slavonia, and the Autonomist Party, which advocated maintaining the autonomy of Dalmatia, opposing the People's Party's demands for unifying Croatia-Slavonia and Dalmatia. The Autonomist Party was also linked to Italian irredentism. By the 1900s, the Party of Rights also made electoral gains in Dalmatia. In Dalmatia, the Autonomists won the first three elections held there in 1861, 1864 and 1867, while those from 1870 to 1908 were won by the People's Party. In 1861–1918 there were 17 elections in Croatia-Slavonia and 10 in Dalmatia.
Session of Sabor on 29 October 1918
Exercising its sovereignty once again on 29 October 1918, the Sabor decided on independence from Austria–Hungary and formation of the State of Slovenes, Croats and Serbs. The council of the newly established state voted to form the Kingdom of Serbs, Croats and Slovenes; however, the Sabor never confirmed that decision. The 1921 constitution defining the new kingdom as a unitary state, and the abolition of historical administrative divisions, effectively ended Croatian autonomy for the time and the Sabor did not convene until the 1940s. The Cvetković–Maček Agreement of August 1939 established the autonomous Province of Croatia, or Banovina of Croatia, in which the Yugoslav government retained control of defence, internal security, foreign affairs, trade, and transport, while other matters were left to the Croatian Sabor and a crown-appointed ban (Viceroy or governor). Before any elections were held, the establishment was made obsolete with the beginning of World War II and the establishment of the Independent State of Croatia which banned all political opposition. In 1942, three sessions of an unelected Sabor were held in the Independent State of Croatia; these were held between 23 February and 28 December 1942, when it was formally dissolved. The assembly had no real power as the state was under the direct rule of (the fascist) Ante Pavelić.
The post-World War II Sabor developed from the National Anti-Fascist Council of the People's Liberation of Croatia (ZAVNOH), formed in 1943. In 1945, ZAVNOH transformed itself into the National Sabor of Croatia, preserving the continuity of Croatian sovereignty. After the war, the Communists ran unopposed in the 1945 elections; all opposition parties boycotted the elections due to coercion and intimidation by the OZNA secret police and the Communist Party, aimed at eliminating electoral dissent. Once in power, the Communists introduced a single-party political system, with the Communist Party of Yugoslavia (from 1952 the League of Communists of Yugoslavia) as the ruling party and the Communist Party of Croatia (from 1952 the League of Communists of Croatia) as a branch party. In January 1990, the Communist Party fragmented along national lines, with the Croatian faction demanding a looser federation. During Communist rule, the Sabor went from a unicameral parliament as specified by the 1947 constitution, to bicameral in 1953, changing again in 1963 to as many as five chambers and then to three in 1974. The constitutional amendments of 1971 established the Presidency of the Sabor, and one of its functions became representing Croatia, as the Yugoslav constituent republics were essentially viewed as nation-states generally surrendering only their foreign and defence policies to the federation; the federal bodies were no longer independent of, but instead formed by, the republics (after 1974 constitution, this role was taken by newly formed Presidency of the Republic elected by the Sabor).
The first political party founded in Croatia since beginning of the Communist rule was the Croatian Social Liberal Party (HSLS), established on 20 May 1989, followed by the Croatian Democratic Union (HDZ) on 17 June 1989. In December, Ivica Račan became the head of the reformed Communist party. At this time, the Communist party decided to cancel political trials, release political prisoners and endorse a multi-party political system. The Civil Organisations Act was formally amended to allow multiple political parties on 11 January 1990, legalising the new parties. By the time of the first round of the first multi-party elections, held on 22 April 1990, there were 33 registered parties. There were single-seat constituencies for half of the seats and a single nationwide constituency (through election lists) for the remaining seats. Still, the most relevant parties and coalitions were the renamed Communist party (the League of Communists of Croatia — Party of Democratic Changes), the HDZ and the Coalition of People's Accord (KNS), which included the HSLS, led by Dražen Budiša, and the HSS, which resumed operating in Croatia in December 1989. The runoff election, open to any candidate receiving at least 7% of the vote, was held on 6 May 1990. The HDZ led by Franjo Tuđman won ahead of the reformed Communists and the KNS. The KNS, led by the former leaders of the Croatian Spring (Savka Dabčević-Kučar and Miko Tripalo), soon splintered into individual parties. On 8 October 1991, Croatia's declaration of independence took effect. The HDZ maintained a parliamentary majority until the 2000 parliamentary elections when it was defeated by the SDP led by Račan. The HDZ returned to power in the 2003 elections, while the SDP remained the largest opposition party.