Under the pressure from the Great powers (British Empire, France, United States), as well as honouring the secret deals that were struck between the Entente and the Kingdom of Serbia, the Kingdom of Serbs, Croats and Slovenes was established and two representatives of Radić's party (by then named the Croatian Common-people Peasant Party) were appointed to the Provisional Representation which served as a parliament until elections for the Constituent could be held. The parties representatives, however, decided not to take their seats.
He was to be held some 11 months until February 1920, just before the first parliamentary elections of the Kingdom of SHS, which were held in November. The result of the November was 230,590 votes, which equaled to 50 seats in the parliament out of 419. Before the first sitting of parliament, after a massive rally held in front of 100,000 people in Zagreb, Stjepan Radić and the CCPP (which after the rally changed the party's name to CRPP - Croatian Republican Peasant Party) held and extraordinary meeting, in which a motion was put forward and voted on that the CRPP will not be part of parliamentary discussions before matters are first resolved with Serbia on the matters of governance, the most sticking issues being the minorisation of the Croatian people and the overt powers of the King with the central government in Belgrade.
In the next parliamentary elections, which were held in March 1923, the stance of Stjepan Radić and the CPP against the central government managed to turn into extra votes. The results of the election were, 70 seats or 473.733 votes, which represented the majority of the Croatian vote in Northern and Southern parts of Croatia, as well as the Croatian votes in Bosnia, as well as Herzegovina.
After his release, Stjepan Radić soon reentered politics, but this was not without problems. On the 23rd of December, the Serb dominated central government declared that the political party CRPP was in contravention of the Internal security law of 1921 in the infamous Obznana declaration, and this was confirmed by King Alexander on the 1st of January 1924, thus arresting the CRPP executive on the 2nd of January 1925, and finally arresting Stjepan Radić on the 5th of January.
In the early 1920s the Yugoslav government of prime minister Nikola Pasic used police pressure over voters and ethnic minorities, confiscation of opposition pamphlets and other measures of election rigging to keep the opposition in minority in Yugoslav parliament. Pasic believed that Yugoslavia should be as centralized as possible, creating in place of distinct regional governments and identities a Greater Serbian national concept of concentrated power in the hands of Belgrade.
After the parliamentary elections in February 1925, the CRPP even with its whole executive team behind bars, and with only Stjepan Radić at its helm, CRPP managed to win 67 parliamentary with at total of 532,872 votes. Even though the vote count was higher than the previous election, the careful carving up of the electoral boundaries by the central government ensured that CRPP received less parliamentary seats. In order to increase his negotiating power the CRPP entered into a coalition with the Democratic party (Demokratska stranka), Slovenian peoples party (Slovenska ljudska stranka) and the Yugoslav Muslim Organisation (Jugoslavenska muslimanska organizacija).
Radić soon resigned his ministerial post in 1926 and returned to the opposition, and in 1927 entered into a coalition with Svetozar Pribićević, president of the Independent Democratic Party, a leading party of the Serbs in Croatia. The Peasant-Democrat coalition had a real chance to end the Radicals' long-time stranglehold control of the Parliament. Previously they had long been opponents, but the Democrats became disillusioned with the Belgrade bureaucracy and restored good relations with the Peasant Party with which they were allies in the time of the Austro-Hungarian Empire. With this arrangement, Stjepan Radić managed to obtain a parliamentary majority in 1928. However, he was not able to form a government. The Peasant-Democrat coalition was opposed by some of the Croatian elite, like Ivo Andrić, who even regarded the followers of the CPP as "...fools following a blind dog..." (the blind dog being Stjepan Radić).
In the Assembly, Puniša Račić, a member of Serbian People's Radical Party from Montenegro, got up and made a provocative speech which produced a stormy reaction from the opposition but Radić himself stayed completely silent. Finally, Ivan Pernar shouted, "thou plundered beys" (referring to accusations of corruption related to him). At this, Puniša Račić drew out a revolver, shot Pernar and went on to shoot Radić and several other CPP delegates. Radić was left for dead and indeed had such a serious stomach wound that he died several weeks later at the age of 57. His burial was massively attended and his death was seen as causing a permanent rift in Croat-Serb relations in the old Yugoslavia.
His assassin was "sentenced" to confinement in a luxury villa in Serbia, where he was tended to by servants.
Following the political crisis triggered by the shooting, in January 1929, King Aleksandar Karađorđević abolished the constitution, dissolved parliament, and declared a royal dictatorship, changing the country into the first Yugoslavia and oppressing national sentiments.
The Ustaše used the death of Stjepan Radić as proof of Serbian hegemony, and as an excuse for their treatment of Serbs, however many leading CPP figures were imprisoned or killed by the Ustashe to whome they were political opponents. The Partisans on the other hand, used this as a recruiting point with CPP members who were disillusioned with the Independent State of Croatia, and latter had one brigade named after Antun and Stjepan Radić in 1943.
The image of Stjepan Radić was used extensively during the Croatian Spring movement in the early 1970s. There are many folk groups, clubs, primary and secondary schools which bear the name of Stjepan Radić. Many Croatian cities have streets, squares in his name and statues of Stjepan Radić are common. The picture of Stjepan Radić appears on the 200 kuna banknote.
In 1997, a poll in Croatian weekly Nacional named Stjepan Radić as the most admired Croatian historic personality.