The historical leader and ideologue of these coalitions was Romano Prodi, Professor of Economics and former leftist Christian Democrat, who invented the name and the symbol of The Olive Tree with Arturo Parisi in 1995.

On 21 April 1996, The Olive Tree won 1996 general election in alliance with the Communist Refoundation Party (PRC), making Romano Prodi the Prime Minister of Italy.

It was the first time since 1946 that the Communists, now gathered in the Democratic Party of the Left, took part in the government of the country and one of their leaders, Walter Veltroni, who ran in ticket with Prodi in a long electoral campaign, was Deputy Prime Minister.

The Democrats of the Left (DS), Democracy is Freedom – The Daisy (DL) and the European Republicans Movement (MRE) decided to form a joint electoral list for the 2006 Italian general election.

The Italian Democratic Socialists (SDI), which were part of the federation and fight the 2004 European and the 2005 regional elections within it, decided not to take part of the joint electoral list for the 2006 general election and, otherwise, to form a common list with the Italian Radicals called Rose in the Fist within The Union coalition.