First period of political action (1912 - 1928)
In the elections on March 11, 1912, he was elected a member of parliament and in 1913 he succeeded Nikolaos Stratos in the Ministry of Marines of the Venizelos government, where he remained for about a year. However, he also became Minister of the Navy a second time during the government of Alexandros Zaimis in 1917.[5]
In 1924, together with other politicians of the time, he founded the "Unionist Progressive Party", in which he was declared leader. The life of this party was short-lived.
General Theodoros Pangalos assumed power in June 1925, and dissolved Parliament in September of the same year. A few months later, in March 1926, after the resignation of the President of the Republic, Pavlos Kountouriotis, who was protesting the arbitrariness of the Pangalos government, Demertzis was nominated, by almost all the parties at the time, as a common candidate to succeed him. The democratic parties finally called for abstention, but to no avail, while Demertzis also did not receive support. Pangalos was comfortably elected President of the Republic, receiving 782,589 votes against Demertzis' 56,126. After the fall of the Pangalos dictatorship, in the elections on November 7, 1926, he was elected Member of Parliament for Athens.
In 1928, he assumed the position of professor of family law at the Law School of Athens. From that year until 1935, he remained away from any involvement with politics, limiting himself to his university duties.[2]
Prime Minister (1935 - 1936)
In 1935 and after the restoration of the Kingdom in Greece, Konstantinos Demertzis, who was not a member of any political party at the time, was invited by King George II to take over as Prime Minister and finally form the " Achron government ", succeeding Georgios Kondylis.[6] Thus, on November 30, 1935, he was sworn in as Prime Minister, keeping the Ministry of Foreign Affairs for himself.[2] The main task of his government was to hold elections, which took place on January 26, 1936, with proven exemplary order. The day after the elections, Demertzis put his resignation at the disposal of the King, but because all the attempts of negotiations made with the political leaders failed to form a government, according to the then constitution, King George II once again resorted to Demertzis in order to form government and then to be judged by the Parliament with a vote of confidence.[2] In the end, this process was not completed, because on the morning of April 13, 1936, Konstantinos Demertzis died from a heart attack.[3] His position was taken by his party's vice-president and former vice-president of his government, Ioannis Metaxas.[6][7]