Delegate at the Great National Assembly at Alba Iulia, which proclaimed the Union of Transylvania and Romania on 1 December 1918. He was then elected president of the Governing Council (the transitional government of Transylvania until 4 April 1920), leader of the Romanian National Party, President of the National Peasants’ Party (from 1926), President of the Council of Ministers (1928-1930). Between 1938 and 1944, he was a vocal opponent of the totalitarian regimes that ruled the country: the dictatorship of King Carol II, the Iron Guard and the military dictatorship of General Antonescu. He played an important role in the preparation of the coup of 23 August 1944, becoming, in the initial stage, a minister without portfolio in the first Sanatescu cabinet. After the communists took power on 6 March 1945, the latter set out to destroy Iuliu Maniu and the National Peasants’ Party, whom they regarded as the main obstacles to the communisation of Romania.
In July 1947, the party was outlawed, and Maniu, Mihalache and the other leaders were arrested, tried and convicted. Almost seventy-five years old at the date of his arrest, Maniu was sentenced, on 11 November 1947, to harsh imprisonment life, convicted of “high treason and espionage for the Anglo-Americans”. He was imprisoned firstly at Galati penitentiary and then, from August 1951, at Sighet. It was here that he would be exterminated, on 5 February 1953. His body was thrown into a mass grave, and thus his remains have remained unidentified to the present day.