The party was elected to power as a minority government in 1963, with Papandreou as Prime Minister. The following year, Centre Union won the repeat elections with nearly 53% of the vote and formed a majority government.
In 1965, after several disputes, King Constantine II forced Papandreou from the office. Almost 45 EK deputies then defected and formed a group of their own in parliament. The King appointed a conservative EK defector, Georgios Athanasiadis-Novas as prime minister. Athanasiadis attempted failed to gain a vote of confidence in parliament, as did Ilias Tsirimokos and the political crisis continued. Finally, King Constantine appointed a third defector, Stephanos Stephanopoulos, who managed a two-vote majority. Political instability continued, however, as National Radical Union (ERE) party leader, Panagiotis Kanellopoulos withdrew his support from the defector government and eventually led an all-ERE government towards the May 1967 elections. EK were poised to win those elections, however, on April 21, 1967, a military coup took place that abolished the constitutional order and ruled by decree.
The Center Union Party was the last Venizelist party to hold power in Greece.
The party nominally continued to exist until 1974, under new leader Georgios Mavros, when its successor Union of the Democratic Center (EΔΗΚ) party was created. Konstantinos Mitsotakis, who had defected the EK in 1965, formed a rival Venizelist party, known as the "Party of New Liberals", which merged with New Democracy in 1978.